refactor(validate): split 1010-line validate.rs into focused submodules

[BryanFRD]

Last of the four-monolith series (#447 config, #448 git, #449 monorepo+versioning).

Split

src/validate.rs (1010 LOC) → src/validate/:

File	LOC	Contents
mod.rs	~99	run entry point + re-exports
source.rs	~189	FileSource trait + LocalSource / GitHubSource / GitLabSource + RemoteProvider + parse_repo_spec + load_config_from_source + parse_config_content
checks.rs	~281	All ten check_* validation passes
result.rs	~59	ValidationLevel, ValidationEntry, ValidationResult, EntryOutput
output.rs	~66	output_result, print_text_result
tests.rs	~315	Tests

Compat

All external use crate::validate::* paths preserved via re-exports. No call-site changes.

Verify

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

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[github-actions]

Benchmark

Details

Benchmark suite	Current: b21b444	Previous: 0b5fe57	Ratio
git_changed_files/100	80217 ns/iter (± 345)
git_changed_files/1000	631161 ns/iter (± 14395)

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