Improve code coverage - #1594

Since the description is empty, for the record this deletes two throw 'unreachable' guards in nodeSet's i===2 and i===3 arms — the only uncovered lines in the file — replacing the i===3 one with assert(xL === 2 || xL === 5) so TSGO still narrows. package.json drops the hardcoded test path from cov; deno.json adds a cov-html task. No version bump, no coverage exclusions, no check weakened.

The assert is provably safe: First<T> (fjs/types/btree/find/types.ts:11-19) permits i===3 only from _FirstLeaf2 (len 2) or _FirstBranch5 (len 5), and find/module.f.mjs:37-49 agrees — done(3) is reachable only at node.length 2 or 5. No reachable input throws.

Behavior verified by differential harness against origin/main: 51,657 tree comparisons (empty and 1..12 ascending+descending, 300 random 1–300-op mixes over a 60-key space with heavy duplicate-insert/replace, 1000-key monotone both directions, triple re-insert, string keys) — 0 mismatches. Coverage of fjs/types/btree/set/module.f.mjs: main 93.97 line / 94.87 branch → head 100/100/100. Old vs new cov command: identical 2793 tests and totals. npx tsc clean; suite 2857/2857.

One convention item: AGENTS.md §5 requires changelog/unreleased/1594.md for a code change and it's absent — every peer coverage PR (#1596, #1597, #1587, #1583, #1576) has one. The empty description also omits the Changelog: section, and ok/xL aren't really commit subjects.

Minor: deno.json:6 anchors module\.f\.mjs$ while cov on line 5 doesn't — harmless, but they now differ.