migrate_v3: content-addressed chunk storage

Status: implemented. migrate_v3 restructures how article text and translations are stored, motivated by the localization model in docs/localization.md (phase 2). Since it is a full migration, it is free to break the current Node.content: str | None layout.

Goals

Minimal change diffs. kanta persists change diffs; editing one paragraph of a long article must not rewrite the whole article string, and a translation refresh must touch only the re-translated chunks.
Fast, simple lookup. Everything heavy lives in flat dict[hash, content] stores; ordering lives in list[hash]. No large nested structures, no deep paths.
Path-independent text. Chunks and their translations are keyed by content hash, not by article path — the same paragraph (or menu title) appearing in several articles is stored and translated once. Moving or renaming an article touches nothing.

Design (chosen: global content-addressed stores)

Original articles are also stored as chunks; everything — originals and translations — lives in flat hash-keyed dicts. Costs accepted: rendering does one dict lookup per chunk (trivial), orphaned hashes need occasional garbage collection, and the editor save path re-chunks server-side (it already diffs). The rejected alternatives: per-article nested LangVersion structures (churn, duplication, whole-string originals) and a hybrid with whole originals plus global translations (keeps the worst change-diff property).

Target layout

class Node(msgspec.Struct, omit_defaults=True):
    ...
    #: Replaces `content: str | None`. None = pure category label;
    #: a list (possibly empty) = a page, as ordered chunk hashes.
    chunks: list[str] | None = None
    #: Primary language of the article (BCP-47 base tag). "" = inherit
    #: (nearest ancestor, front page last, site default "en" final).
    language: str = ""
    #: Chunk hashes the editor marked "do not translate" (always served
    #: from the original). Presence-keys, value always True.
    no_trans: dict[str, True] = {}
    #: Languages this article is available in (besides its primary
    #: language). Presence-keys, value always True — rendering, language
    #: selection and hreflang alternates read this set instead of probing
    #: the trans store chunk by chunk. Maintained by the writers (see
    #: "Language index maintenance" below).
    langs: dict[str, True] = {}

class Data(msgspec.Struct):
    ...
    #: All original-language text, content-addressed: blake3(normalized,
    #: digest 16) hex -> Markdown chunk. Shared by every article.
    chunks: dict[str, str] = {}
    #: Machine translations: f"{chunk_hash}:{lang}" -> translated Markdown.
    #: Also used for node titles (hash of the title text).
    trans: dict[str, str] = {}
    #: User override patches per article and language:
    #: f"{path}:{lang}" -> ordered patches (see localization.md).
    patches: dict[str, list[Patch]] = {}

Notes:

Article paths never carry a leading slash in the DB or in lookup keys ("docs/setup", front page ""); the leading slash is added only when building hrefs. migrate_v3 audits existing stored paths (translation keys, analytics references, any path-valued fields) and normalizes them.
Titles are chunks too, by hash only: the nav renderer looks up trans.get(f"{hash(node.title)}:{lang}"). No separate title storage; editing a title invalidates its translations automatically.
Per-hunk options live in two places: inherent options are derived at chunking time (code fences and HTML blocks are marked no-translate without storing anything); editor-set flags are node.no_trans (keyed by chunk hash, so a heavy edit silently drops the flag — acceptable and self-healing).
Patch payloads stay inline in Patch.hunks — patches are small by construction (minimal server-computed diffs). If a pathological case shows up, hunks can be hash-stored later without schema pain.

Language index maintenance (`node.langs`)

node.langs is a denormalized index over the trans/patches stores so that article rendering, select_language's availability check, and hreflang alternate links never enumerate chunks. It is written by whoever writes translation data, in the same transaction:

Translator job: after writing trans[f"{h}:{lang}"] entries for an article's chunks (or its title), set node.langs[lang] = True.
Translated-view save: appending the first patch for f"{path}:{lang}" sets node.langs[lang] = True (patches alone make the version exist).
Removals: deleting a patch or GC'ing translations re-derives the key: keep lang if any trans entry for the article's current chunks/title or any patch remains, otherwise drop it. Stale langs keys are benign (an advertised language that renders as the original), so removal can lag.

Render / save pipeline (summary)

Render: text = "\n\n".join(chunks[h] for h in node.chunks) for the original; for language L (only ever attempted when L in node.langs), per chunk trans[f"{h}:{L}"] unless missing or h in node.no_trans, falling back to chunks[h]; then apply patches.get(f"{path}:{L}", []) in order (per-hunk, best effort); then markdown.render as today. All of this assembles the Translation the phase-1 plumbing already consumes.
Availability: available_languages(path) = sorted(node.langs); hreflang alternates and ?lang= handling use exactly this set.
Save (primary language): server re-chunks the submitted Markdown, inserts new hashes into Data.chunks, replaces node.chunks. Unchanged chunks keep their hashes — only genuinely new text lands in the diff.
Save (translated view): diff against the served hybrid, append a Patch under patches[f"{path}:{lang}"]; node.chunks untouched.
Invalidate: any write to chunks / trans / patches calls _invalidate_pages().

migrate_v3 steps

Walk menu; for every node with a string content: chunks = chunk_markdown(content); write each into the new chunks store; replace the field with the hash list (None stays None).
Initialize empty chunks / trans / patches stores.
Normalize stored paths: strip leading slashes anywhere paths are keys or values.
language, no_trans and langs need nothing — struct defaults cover them (langs starts empty; the translator job fills it as translations land).

Chunking must be deterministic and shared with render/save, so chunk_markdown + chunk_key live in pagerite/i18n.py (or a small pagerite/chunks.py) and are imported by both migrations.py and views.py/app.py.

Implementation notes (deviations from the plan above)

Chunking lives in pagerite/chunks.py; hashing uses the blake3 package (already a dependency) with a 16-byte digest (hexdigest(16)).
Translation.titles stayed keyed by node path (phase-1 shape, views untouched): get_translation builds it by walking the menu with the same per-title trans[f"{chunk_key(node.title)}:{lang}"] lookups.
Insert hunks anchor on the whole preceding block (not just its tail) — a stronger, simpler search context.
make_patch diffs with SequenceMatcher(autojunk=False) so patches are deterministic (popular lines like blank separators never become junk).
Step 3's path normalization is a no-op in practice: the only path-keyed store (patches) starts empty at v3; analytics paths live outside the kantadb. The code still strips leading slashes defensively.

Garbage collection (later, manual or idle-time)

Orphaned entries accumulate: chunks no longer referenced by any node.chunks/node.title, translations whose chunk hash is orphaned, patch hunks that never match. All are harmless (never read). A GC pass is a single tree walk collecting live hashes, then deleting the rest from chunks and trans; patches whose every hunk is stale get pruned. Not part of migrate_v3.

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