refactor: split monorepo.rs and versioning.rs into focused submodules

[BryanFRD]

Why

Two more monoliths trimmed down following #447 (config) and #448 (git):

File	Before	After
src/monorepo.rs	1792 LOC, single file	src/monorepo/ — 8 files, biggest is run.rs (1125 LOC, isolates the run_release_logic pipeline)
src/versioning.rs	861 LOC	src/versioning/ — 4 files

monorepo split

File	LOC	Contents
mod.rs	12	Re-exports check and release
check.rs	~42	The check entry point
release.rs	~124	release, cleanup_failed_release_attempt, MAX_RELEASE_REGENERATE_ATTEMPTS
run.rs	~1125	run_release_logic — the core pipeline both check and release call
preview.rs	~90	Preview-comment posting (PR/MR forge integration)
util.rs	~97	tags_for_package, pick_higher_semver, collect_dirty_files, auto_stage_new_files, is_package_touched
types.rs	~23	CheckCommit, CheckPackage, CheckResult
tests.rs	~195	Tests

versioning split

File	LOC	Contents
mod.rs	~58	Top-level entry points: bootstrap_version, compute_next_version, bump_version, truncate_version
strategies.rs	~98	Per-strategy bumpers: bump_semver, calver_version, calver_seq_version, bump_sequential, bump_zerover
detect.rs	~107	Tag classification: strip_tag_prefix, TagClass, classify_tag, detect_strategy_from_tags
tests.rs	~526	Tests

Compat

All external imports unchanged (use crate::monorepo::*, use crate::versioning::* keep working via re-exports).

Verification

• cargo test --bin ferrflow → 598 passed
• cargo clippy --workspace --all-targets -- -D warnings → clean
• cargo fmt --check → clean

Conflict

refactor/git-split-into-modules (#448) and fix/push-tags-shell-out-libgit2-revwalk-bug may need to be rebased on top of this. Suggested order: merge those two first if possible, then rebase this.

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[github-actions]

Benchmark

Details

Benchmark suite	Current: e5c5e51	Previous: 0b5fe57	Ratio
git_find_tag/100	1355511 ns/iter (± 38746)
git_find_tag/1000	13933470 ns/iter (± 208995)

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