Add coverage gate and performance smoke check

[krkarma777]

Closes #20

Summary

• CI test job now runs under c8 with enforced floors (statements/lines 97, branches 92, functions 99); measured coverage is 99.76% statements / 96.89% branches / 100% functions
• Added a test covering the Intl.Segmenter-unavailable error path (the only meaningful uncovered line); remaining uncovered lines are defensive-unreachable guards in the Myers core
• New `npm run perf:smoke` runs 4 scenarios against the built dist with ceilings 50-100x above measured medians — catches accidental complexity/allocation regressions while staying green on noisy shared runners; wired into CI after the build
• Static coverage badge added to the README badge row; CHANGELOG finalized as 1.2.0

Test Plan

• `npm run test:coverage` passes floors locally (59 tests)
• `npm run perf:smoke`: word 1.23 ms / char 0.65 ms / heuristic worst 8.25 ms / exact worst 266 ms — all under ceilings