[leas code] Share usage event attribution

[nwparker]

Summary

• Shares Codex/OpenCode worktree canonicalization, local-day attribution, containment, project keys, and fallback labels.
• Reuses generic session/daily record shapes and removes six unused provider breakdown exports.
• Makes attribution synchronous while preserving provider-specific event fields and ownership ordering.
• Fixes POSIX/WSL case handling on Windows, drive/UNC containment, forward-slash UNC recognition, non-native realpath resolution, and POSIX relative backslashes.
• Advances the local cache schemas so old projections are reparsed while preserving the user's tracking opt-in.

Size

• 376 additions, 660 deletions: 284 fewer lines overall
• Production code: 182 additions, 463 deletions: 281 fewer lines

Validation

• 11 focused Vitest suites / 72 tests
• pnpm run typecheck
• pnpm run build:electron-vite
• Focused oxlint and oxfmt checks
• Changed-code quality, reliability, max-lines, and diff checks
• Three independent reviews found no remaining P0-P2 correctness, migration, cross-platform, remote, or security findings

Compatibility

No RPC/wire, persisted record shape, dependency, network, auth, or UI changes. Cache schema bumps only invalidate local derived projections.

No visual change.

Stack

Stacked on #13558 (nwparker/leas-code-usage-store-lifecycle). Retarget to main after the base lands.

The bounded-memory usage work should restack scanner call sites onto attributeUsageEvent while preserving provider ownership order and lifecycle writer/generation-veto behavior.