Summary

Add a positive-control test set to mod tests_compiler_rejects in
bootstrap/src/compiler.rs: where the existing negative tests (added in #1123 / variant Q)
pin what the declaration parser rejects and drops, these pin what it accepts and lowers.
Pure test additions; no production code changes.

Motivation

After #1123 we have good coverage of rejection/recovery behavior at the declaration level,
but the acceptance side is comparatively under-pinned. A regression that silently stopped
lowering a valid construct (a const, an fn body, a struct) would not be caught by the
negative tests alone. This adds the mirror set so both directions are locked in.

Following the project rule, each test was written by empirically observing the current
behavior first, then asserting it -- the new tests characterize today's behavior, they do
not endorse or prescribe it.

New tests (5)

• accepts_multiple_consts -- two consts (A=7, B=9) both lower.
• accepts_fn_with_body -- a function with a body (id=x) lowers.
• accepts_mixed_const_and_fn -- a const plus a function both lower.
• accepts_struct_declaration -- a struct declaration lowers (its name appears in output,
  no TODO marker emitted).
• lowers_only_first_of_two_modules_characterization -- with two modules, only the FIRST is
  lowered (emitted module count == 1; the second module's symbols are absent). This is a
  characterization of current behavior, explicitly named as such.

Verification

• tests_compiler_rejects count: 16 -> 21.
• Full suite: 1463 passed, 0 failed, 0 regressions.

L6 gf16 SSOT untouched; catalog stays 83; no gen/ edits; ASCII-only added lines; no quality
claim added.