Feat: Establish structural and text fallback baseline

[marco0560]

Summary

Provide Tier B structural and Tier C text retrieval for unsupported text files without claiming primary-language semantics.

Acceptance criteria

• Section/declaration-oriented structural chunks where safe.
• Deterministic text indexing and retrieval for unsupported files.
• Explicit quality tier and provenance in results.
• No fragment recovery or primary-parser replacement.

Unblocks

• Support optional fallback analyzers for whole-file and fragment-level parse gaps #1