fix: validate top module first

[bananaplain]

When more than one module of a requires chain had an error, plain-parser check and codeplain --dry-run reported different first errors. Given top requiring middle, middle requiring base, and an undefined concept in both middle and base, plain-parser reported base's and codeplain middle's.

plain-parser validated each required module as the requires walk reached it, deepest first, and checked the top module's structure after the walk. On the other hand, codeplain checks the top module's structure before the walk and runs its remaining checks after it. It then validates each required module in a fresh parse, top-down, in listed order.

Change

• validate_and_marshall_module is replaced by two functions, with no behaviour change:
  • validate_module_structure: required concepts defined, functional specs present and non-empty, implementation reqs present.
  • validate_and_build_module_tree: exported definitions merged, acceptance tests, linked resources, code variables, concept validation, marshalled tree.
• The top module's structure is checked before the walk.
• The walk parses and records each required module as a RequiredModule: parse result, the exports each receives, its own exports, code variables. Circular, fixed-order and parse errors are raised during the walk, as before.
• After the top module, recorded modules are validated top-down in listed order, each once. Accept/reject, marshalled trees, and the deepest-first chain from parse_module_chain are unchanged. Each file is still read once.
• New tests in tests/test_requires.py pin each ordering rule with two broken modules, an accepted chain listing one module twice (requires: [base, base]), a required module without functional specs, a top-module concept error reported before a required module's missing functional specs, and that each module's tree is built once, top module first.
• Compared with codeplain origin/main on 10,000 random requires graphs: no differences in verdict, first error or tree. Before this change, 329 first errors differed.

Release

• Ships as 0.1.4.
• One fixture reports a different error than 0.1.3: requires/circular_requires_sub has missing implementation reqs and a circular require, and now reports the missing implementation reqs first, as codeplain does.
• plain-forge users need uv tool upgrade plain-parser; plain-forge update does not upgrade it.

[hisenb3rg]

LGTM 👍