fix: gate flatbuffers regeneration behind REGEN_FLATBUFFERS

[netsirius]

Problem

rust/build.rs ran flatc --rust -o src/generated ... on every build, rewriting the checked-in generated files with whatever flatc is installed locally. A mismatched flatc turned a plain cargo check into ~4k dirty tracked lines, which twice got swept into commits (#72, reverted by #73; the initial commit of #83).

Solution

Regeneration is now opt-in (option 2 from the issue):

• Normal builds never touch src/generated/ and don't require flatc — the checked-in files are the source of truth.
• After editing a schema, run REGEN_FLATBUFFERS=1 cargo build and commit the .fbs and regenerated .rs together (documented in CONTRIBUTING.md). REGEN_FLATBUFFERS=0 or empty counts as unset.
• Since regeneration is now explicit, it fails loudly: missing flatc or a nonzero flatc exit panics the build script instead of silently succeeding with stale files.

Testing

Scenario	Result
cargo check (no env var)	src/generated/ untouched, flatc not required
REGEN_FLATBUFFERS=0 cargo build	no regeneration
REGEN_FLATBUFFERS=1 cargo build	regenerates, output matches checked-in files
REGEN_FLATBUFFERS=1 without flatc	build fails with a clear message

Possible follow-up: a CI job regenerating with a pinned flatc that fails on diff.

Fixes

Closes freenet/freenet-core#4747