fix(packages): migrate older DiskCache schemas to add leases.refcount

[zackees]

Summary

Fixes #124. Any DiskCache::lease() call against a production cache DB created before the leases.refcount column was added errors with no such column: refcount, blocking fbuild lnk pull and every other lease-consuming code path.

Approach — option B (versioned migrations)

Replaces the single-step migrate_v1 with an append-only, idempotent migration framework keyed by stable ids in a new schema_migrations table:

• m001_initial_schema — original tables + indexes, all CREATE TABLE IF NOT EXISTS so it's safe on databases that already have them.
• m002_add_leases_refcount — ALTER TABLE leases ADD COLUMN refcount INTEGER NOT NULL DEFAULT 1; guarded by a PRAGMA table_info(leases) check. No-op on caches that already have the column from m001.

Each migration runs inside its own transaction and is skipped if its id is already present in schema_migrations. Future deltas are append-only. The legacy cache_meta.schema_version key is still mirrored for tooling that inspects it, but the authoritative source of truth is now the migrations table.

Chose option B over option A because the issue itself flags option 2 as the longer-term clean answer and the framework cost is small (~60 LOC) compared to re-adding a migration infrastructure the next time a column gets added.

best_effort_lease revert

This PR also reverts the best_effort_lease() workaround introduced in #119's lnk/resolver.rs. Since every opened cache now has leases.refcount guaranteed by the migration, the lease is once again a hard requirement — cache.lease(entry).map_err(map_cache_err)?. Revert is in the same commit as the migration: 90d18d0.

Files touched

• crates/fbuild-packages/src/disk_cache/index.rs — migration framework + MIGRATIONS list + leases_has_refcount helper + 3 new unit tests.
• crates/fbuild-packages/src/lnk/resolver.rs — remove best_effort_lease, use cache.lease()? directly.
• crates/fbuild-packages/tests/disk_cache_schema_migration.rs — new integration test seeding a legacy schema and asserting DiskCache::open_at + cache.lease() succeed end-to-end.
• crates/fbuild-packages/tests/README.md — new (required by repo's readme-guard hook).

Test plan

• uv run cargo test -p fbuild-packages — all 391 unit tests + 2 new migration integration tests + 4 lnk_e2e tests pass.
• uv run cargo clippy --workspace --all-targets -- -D warnings — clean.
• New test_legacy_schema_missing_refcount_is_migrated reproduces the exact bug scenario (pre-migration DB → open → lease) before the fix and passes after.
• test_migrations_idempotent_across_reopens guards against re-running ALTER TABLE on second open.
• Fresh-DB regression guard (fresh_cache_lease_still_works) confirms no regression for new users.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 22ac8a1c-bc12-43d8-9701-e2a3a938a10a

📥 Commits

Reviewing files that changed from the base of the PR and between aed0649 and 90d18d0.

📒 Files selected for processing (4)

• crates/fbuild-packages/src/disk_cache/index.rs
• crates/fbuild-packages/src/lnk/resolver.rs
• crates/fbuild-packages/tests/README.md
• crates/fbuild-packages/tests/disk_cache_schema_migration.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/disk-cache-schema-migration

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