Improve documentation, comment uniformity, and test coverage

[Copilot]

Addresses the open items from the documentation and testing issue: uniform inline comments, extended metacommand test coverage, and subdirectory READMEs.

Comment uniformity

• Fixed typo singla → single and missing period in IMP/Lexer.hs
• Removed dead commented-out code in REPL/State.hs
• Capitalized all -- INFO: / -- CHECK: comment bodies consistently across the codebase (14 occurrences)
• Added missing periods to Haddock doc comments in REPL/Execute/Browser.hs

Test suite — metacommand parse & execution (60 → 102 tests)

Added REPL.Meta, REPL.Preset, and REPL.State as other-modules (+ Paths_impli autogen) in the test suite to enable REPL-level testing without touching the library API.

Two new test groups in test/Spec.hs:

• metaParseTests — 33 cases covering all Command variants (including single-letter abbreviations), Aspect, Option, Level, and Element parsing via parser "test" input @?= Right expected
• metaExecTests — 10 cases running reset/set actions through the REPL IO monad and asserting the resulting Store fields

Subdirectory READMEs

File	Contents
src/IMP/README.md	Module table + lexer→parser→semantics pipeline diagram
src/IMP/Semantics/README.md	Natural vs. structural semantics, function signatures, tradeoffs
src/REPL/README.md	REPL m monad structure, dispatch flow, metacommand overview
src/REPL/Execute/README.md	Native (haskeline/InputT IO) vs. browser (WASM FFI/IO) backend comparison
web/src/README.md	Frontend source files (App.tsx, impli.ts, util.ts, Unsupported.tsx) and WASM startup flow

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve documentation and testing</issue_title>
<issue_description>- [x] Typst paper of implemented derivation rules
- [x] docs/IMP.typ compiled to docs/IMP.pdf with GitHub action in build workflow on push that changes the source file
- [x] Add docs/IMP.pdf to .cabal extra docs files

• Refine Haddock documentation comments
• General comments
• Extend test suite
  • Also test metacommands' parse and execution
• Add READMEs to relevant subdirectories in src/ and web/
  • For instance in Semantics/, REPL/, IMP/, etc. </issue_description>

Comments on the Issue (you are @copilot in this section)

@bfeitknecht Make comments uniform. Capitalization, punctuation.

• Fixes Improve documentation and testing #12

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[bfeitknecht]

@copilot Also add a README to web/.

[Copilot]

@copilot Also add a README to web/.

Added web/src/README.md in dbb2fce, covering the four source files (App.tsx, impli.ts, util.ts, Unsupported.tsx), the WASM startup flow, and a link back to the top-level web/README.md for build/deploy details.