fix(explore): retain broad receiver and requested source evidence

[bompus]

Summary

Stacked on #1841 (literal seeds). This adds the follow-up that keeps compound explore queries working when the question names a broad receiver or asks for a specific region of a file:

• src/mcp/explore-source-ranges.ts — new requestedSourceRanges() scores source ranges inside an already-selected file against the question's terms, camelCase identifiers, and seed literals. It never adds graph nodes or edges — it is evidence inside a file the query already chose. Test paths and Vue files get dedicated region handling, and the component's own basename is subtracted from the question so a name that identifies the file doesn't seed every line inside it.
• src/mcp/tools.ts + src/extraction/* — retain broad-receiver and requested-source evidence through the explore path so readers/writers stay attached instead of degrading to bag-of-words FTS.

Dependency

Contains the full contribute/literal-seeds branch (#1841) via merge commit 4d0f0778; the diff vs main will shrink to just this change once #1841 lands.

Validation

Regression checks and the upstream-contribution validation runs are recorded in the branch's docs(explore) commits (bca1cc01, b56cd7ee). Not rerun against latest main for this draft.

[bompus]

Rebased onto current main as a single clean commit (the old branch had grown the fork's history; this is now just the feature). No source conflicts beyond the changelog — #1849/#1862 touch different regions of the files this PR changes.

Verified on the rebased tree: tsc clean, the PR's 7 test files green (248 passed in the two compiled-gated files included), including literal-compiled-index against a fresh dist build.

[bompus]

Closing so it doesn't sit in your review queue. The change lives on the fork as 245d00d4. I've asked in #2332 whether you want any of the fork's explore work, and can resend this in smaller pieces if you do.