fix(lower): bare-call resolver binds same-name flat authors per source [0102c]

Third of four fix-0102 sub-steps — the behaviour fix for NORMAL call sites.
Adds THE bare-name resolver `resolveBareCallee(name, caller_file)` over
fix-0102a's `module_fns` + `flat_import_graph` and routes the primary call
path through it:

- own-author wins: a file's bare call to a name IT authors binds its OWN
  author, not the first-wins merge winner. (When the winner already is the
  caller's own — every single-author and first-importer case — the resolver
  returns `.none` so the existing path binds it byte-for-byte.)
- a bare call to a name two or more FLAT imports both provide is `.ambiguous`
  and rejected with a loud diagnostic ("declared by multiple imported
  modules — qualify the call"); a namespaced author never collides.
- a single flat-reachable author that differs from the winner binds that
  author; otherwise `.none`.

The resolved shadow author lowers into its OWN FuncId via fix-0102b's
identity-addressable `lowerFunctionBodyInto` (shared `bareAuthorFuncId`
helper, also used by `lowerRetainedSameNameAuthors`). Only plain free
functions route — generic / comptime / foreign / builtin authors and any
scope-mangled / UFCS-aliased / locally-shadowed name fall straight to the
existing dispatch, so single-author / local / std / qualified resolution is
unchanged (full example suite stays green, including bundle.sx and the
comptime format/pack examples).

Examples 0722 (flat file per-source bind), 0723 (flat vs namespaced, no false
ambiguity), 0724 (ambiguous → diagnostic), 0725 (flat directory per-source
bind), 0727 (user namespace literally named __m0). Each fails on
wt-fix-0102-base (first-wins mis-bind / no diagnostic) and passes here. The
fix-0102b unit test now calls a per-module wrapper (main can't bare-call the
2-author name) and asserts the resolver's three variants directly.

Gate: zig build, zig build test (400/400), bash tests/run_examples.sh
(462 passed) all green.

examples/expected/0725-modules-flat-dir-same-name.exit

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