Improve test coverage from 92.3% to 100% statements

[Copilot]

Several code paths in basic-tts.js had no test coverage: the checkVoices attempts normalization logic, the speak() null-utterance rejection path, and the AMD define export block.

New test cases

checkVoices attempts normalization (lines 132, 136)

• String attempts (e.g. "2") → parsed via parseInt
• Non-numeric string (e.g. "abc") → parseInt yields NaN → resets to default
• Negative number → resets to default

speak() with null utterance (line 214)

• Voices available but voice prop doesn't match any → getUtterance returns null → rejects with "invalid voice" message

AMD define block (lines 266–271)

• Mocks global.define with define.amd = true via jest.resetModules() to force re-evaluation of the module-level guard
• Verifies both define("basic-tts", ...) and anonymous define(...) are called with correct factory functions
• Verifies define is not called when define.amd is absent

Coverage delta

Metric	Before	After
Statements	92.3%	100%
Functions	90.9%	100%
Lines	92.3%	100%
Branches	80.4%	91.3%

Remaining uncovered branches (lines 40–41, 258) are environment-detection guards (typeof window for browser context, Node module.exports false branch) not reachable in Jest.