fix(search): snippet always fills the full context window

[ashishbhateja]

What

The snippet() function in src/search.js had an off-by-half bug: when a matched term appeared near the start or end of the text, the context window was clipped to half of contextChars instead of the full width.

Before (simplified):

const start = Math.max(0, idx - half);      // clamped to 0 near the start
const end   = Math.min(len, idx + half);    // only half the window remained

When the match was at idx = 0, the window was [0, 60] — 60 chars instead of the intended 120.
When the match was near the end (e.g. idx = len - 5), the window was [len - half - 5, len] — again roughly half.

After:

const start = Math.max(0, Math.min(idx - half, text.length - contextChars));
const end   = Math.min(text.length, start + contextChars);

The window slides to compensate for edge clamping, so it always spans contextChars characters when the text is long enough. The matched term is guaranteed to remain visible: start ≤ idx by construction.

Tests

Two new cases added to scripts/smoke.mjs (now 21 checks, up from 19) that would have caught this bug:

• snippet fills the full context window when the matched term is near the start
• snippet fills the full context window when the matched term is near the end

All 21 checks pass. npm run build succeeds.

Issue

Advances #3 — specifically the "result snippets" part: a term that leads an article's summary or body now shows the reader a useful 120-char window instead of a half-width stub.

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