feat: structured debug log prefixes

[maastrich]

Summary

At --log-level debug the hasher now emits one line per event, each starting with a bracketed tag at column zero so users can filter with `grep`:

• `[hash]: ` — a file's content was read and its sha256 computed.
• `[import]: -> "" -> ` — a static import was resolved. Unresolved specifiers show `` in the third slot.
• `[skip]: ` — a resolved path was skipped because it lives in `node_modules`.

hashup -l debug 2>&1 | grep '^\[hash\]:'
hashup -l debug 2>&1 | grep '<unresolved>'
hashup -l debug 2>&1 | grep -c '^\[skip\]:'

Test plan

• New `tests/debug-logs.test.ts` — asserts each prefix is present on a fixture with one hashed file, one resolved import, and one node_modules skip; asserts every debug line starts with one of the three prefixes at column zero; asserts `warn` suppresses all three
• `vp test` — 120/120 passing
• `vp check` clean
• Smoke tested against bfront webapps/lcm — 2056 `[hash]`, 6402 `[import]`, 1209 `[skip]` events, 115 MB peak RSS, ~1 s wall time

Docs

`docs/guide/how-it-works.md` → new "Debug-level prefixes" section with the table and `grep` recipes.

🤖 Generated with Claude Code

[github-actions]

📖 Docs preview: https://maastrich.github.io/hashup/branches/pr-20/