(feat): cache external (plugin-generated) file content in the defs cache

[eytan-starkware]

Serialize each external file's VirtualFile content into the crate cache blob, keyed by a
portable, parse-free identity (ExternalFileContentKey), and serve it from ext_as_virtual
before falling back to module_sub_files. A loaded crate cache can then materialize
plugin-generated files without re-running plugins. The lookup depends only on the blob
(no module-data edge), so it never re-enters cache loading.

[reviewable-StarkWare]

This change is 

[eytan-starkware]

• (opt): keep syntax node ids stable across reparses #10105
• (feat): cache external (plugin-generated) file content in the defs cache #10164 👈 (View in Graphite)
• (refactor): read and write crate cache blob sections via CacheBlobReader #10167
• main

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[cursor]

PR Summary

Medium Risk
Touches core cache blob layout (new section shifts indices) and ext_as_virtual resolution during cache load, where incorrect keys or misses can panic or serve wrong generated code; behavior is guarded but stale caches may need regeneration.

Overview
Adds a new first blob section (EXTERNAL_CACHE_SECTION) so plugin-generated (external) file bodies are stored in the crate cache and later served without re-running macros. Section order becomes [external, def, semantic, lowering]; existing def/semantic/lowering readers skip the new prefix via section indices.

On save, caching external FileIds also records VirtualFile payload in an ExternalFileKey → ExternalFileContentCached table (keys are parse/plugin-free: on-disk path, virtual parent+name+content hash, or generated parent+offset+name). On load, external_file_contents reads only that section from the blob (keyed by blob_id, not module data) so ext_as_virtual does not pull in defs cache and create salsa cycles.

ext_as_virtual_impl now returns cached content when present; for cache-backed crates with a loaded blob, a miss asserts instead of falling back to module_sub_files (that fallback would re-enter loading). Non-cache-backed crates still use the plugin path.

Lowering artifact writing prepends the external table; cache tests cover #[derive(Drop)] at crate root and in a submodule.

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[orizi]

@orizi partially reviewed 4 files and made 1 comment.
Reviewable status: 4 of 5 files reviewed, 2 unresolved discussions (waiting on eytan-starkware and TomerStarkware).

---

crates/cairo-lang-lowering/src/cache/test.rs line 90 at r3 (raw file):

/// strings stand in for parent `FileId`s, which are interned per-db and thus not comparable.
#[derive(Clone, Debug, PartialEq)]
struct ExternalFileSnapshot {

re-valitate.

[orizi]

@orizi reviewed 5 files and all commit messages, made 1 comment, and resolved 1 discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).

[graphite-app]

Merge activity

• Jun 28, 11:19 AM UTC: Graphite rebased this pull request, because this pull request is set to merge when ready.

[orizi]

@orizi reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).