Replace parser surface with a Parsec compatibility layer

[falgon]

Summary

• replace the Megaparsec dependency with parsec and provide local Text.Megaparsec* compatibility modules
• adapt parser combinators and CLI error handling to the compatibility layer without changing existing tests
• preserve diagnostic formatting by restoring trailing newlines in errorBundlePretty and avoiding bogus source-line fallback at EOF-after-newline

Verification

• STACK_ROOT=/tmp/htcc-stack3-root stack --docker --docker-image htcc-stack-bullseye:latest --docker-run-args='--platform linux/amd64' --docker-stack-exe download build --test --no-run-tests
• STACK_ROOT=/tmp/htcc-stack3-root stack --docker --docker-image htcc-stack-bullseye:latest --docker-run-args='--platform linux/amd64' --docker-stack-exe download test --test-arguments 'components'
• STACK_ROOT=/tmp/htcc-stack3-root stack --docker --docker-image htcc-stack-bullseye:latest --docker-run-args='--platform linux/amd64' --docker-stack-exe download test --test-arguments 'subp'
• STACK_ROOT=/tmp/htcc-stack3-root stack --docker --docker-image htcc-stack-bullseye:latest --docker-run-args='--platform linux/amd64' --docker-stack-exe download test --test-arguments 'self'
• docker-path verification via linux/amd64-pinned docker compose up --build --abort-on-container-exit --exit-code-from htcc against the generated /tmp/htcc/asm/*.s

Closes #2.