chore: migrate src/ off mod.rs to modern module-file style

[iheitlager]

Description

src/ still uses the legacy foo/mod.rs module layout in 6 places
(vfs, record, parser, schema, btree, pager) — ff mod.rs | wc -l reports 7,
but the 7th (tests/spike/.../lemon-rs) is vendored spike code, out of scope.
Goal: ff mod.rs | wc -l == 0 for src/, permanently enforced.

Complexity

Estimate: small
Reasoning: Mechanical git mv foo/mod.rs foo.rs per module (6x), no
logic changes; plus one Cargo.toml lint line and one Makefile gate target.

Scope

1. For each of vfs, record, parser, schema, btree, pager: git mv src/<m>/mod.rs src/<m>.rs. No content changes, no behavior change.
2. Add self_named_module_files = "deny" to [lints.clippy] in
   Cargo.toml (clippy restriction lint — forbids foo/mod.rs in favor of
   foo.rs), matching the existing deny-list style (unwrap_used,
   expect_used, etc.) already there.
3. Add a mod-files gate to the Makefile (alongside grammar-drift,
   mvl-limit) that fails if any mod.rs exists under src/, and wire it
   into whatever aggregate gate target composes those checks.

Acceptance Criteria

• find src -name mod.rs returns nothing
• cargo clippy --all-targets -- -D warnings catches a reintroduced
  mod.rs via self_named_module_files
• make mod-files (or equivalent) fails on a reintroduced mod.rs and
  passes otherwise; wired into CI same as lint/grammar-drift
• tests/spike/.../lemon-rs left untouched (vendored, out of scope)

Additional Notes

Belt-and-suspenders: the clippy lint alone should already catch this in
make lint; the Makefile target is a cheap, dependency-free backstop for
anyone running gates without clippy (and self-documents the goal each time
gates run).