Function readBase

Read and parse a .base file.

v3.7.12 H2 — path normalization parity with readCanvas / readPdf:

• path is required (rejects empty string)
• extension auto-appended (Books → Books.base)
• non-.base paths rejected (caller can't accidentally read a .md file through this surface and trigger parser errors)
• resolves through vault.resolveInside (path-traversal guard)
• stat() checked early — fail fast on missing/excluded files
• returned path is the canonical vault-relative form, so callers can re-issue requests by the same key regardless of how they spelled the input