Harden canonical field parsing

[kiki830621]

Summary

• Replace canonical 5-run parser fragment joining with lazy per-run fragment scanning.
• Document the baked-form cached-run discriminator with a named helper.
• Add baked/canonical invariant tests and a 100k-run defensive scan regression test.

Tests

• swift test --filter Issue104FieldParserCanonicalFormTests
• swift test

Refs #32, #33