chore: mark unsupported code paths with coverage:ignore

[AquibAquil]

Addresses part of #263.

What

Adds // coverage:ignore-* markers to code paths documented in-source as intentionally unsupported or unreachable:

Whole-file ignores — lib/src/impl_stub/ (14 files)

Marked with // coverage:ignore-file. This directory is loaded only via conditional import when neither dart.library.ffi nor dart.library.js_interop is available (see lib/src/webcrypto/webcrypto.dart:27). No real Dart runtime satisfies that condition, so every throw UnimplementedError('Not implemented') in the directory is unreachable at runtime by design.

Block ignores — 4 files in impl_ffi/, impl_js/, and boringssl/

Wrapped with // coverage:ignore-start / // coverage:ignore-end:

• Three throw UnsupportedError('192-bit AES keys are not supported') sites in lib/src/impl_ffi/impl_ffi.aes_common.dart (documented in-source as intentionally unsupported, see https://crbug.com/533699).
• The unsupported-platform fallback throw in lib/src/boringssl/lookup/utils.dart (fires only on operating systems outside {Windows, Linux, macOS}).
• Five "This should never happen!" curve fallbacks in lib/src/impl_ffi/impl_ffi.ec_common.dart (×3), lib/src/impl_ffi/impl_ffi.ecdsa.dart, and lib/src/impl_js/impl_js.utils.dart — dead-code fallbacks after exhaustive enum switches.

Why

Per #263, these are candidates where "we don't want coverage" — code paths intentionally chosen not to test rather than defensively cover.

Skipped intentionally

• requireSubtleCrypto (lib/src/crypto_subtle.dart:82) — the example cited in the issue. Now has tests at test/crypto_subtle_test.dart:128 and :141, so ignore markers would silence real coverage. Left alone.
• throw UnimplementedError('Only supports 65537 and 3 for now') (lib/src/crypto_subtle.dart:44) — genuine TODO for future work, not an intentional gap. Left visible.

Verification

• dart format --output none --set-exit-if-changed . passes.
• dart analyze --fatal-warnings . passes.
• Grep confirms no existing tests exercise the ignored throws (except requireSubtleCrypto, which is why it's excluded).
• Local test run was not possible on Windows: the native asset build fails with CMake Error at CMakeLists.txt:19 (project): Generator Ninja does not support platform specification, but platform x64 was specified. Working around it locally was out of scope for this change; relying on CI to validate.