fix(core): emit compile_error for --no-default-features (#659)

[gustav-fff]

Closes #659

Root cause

fff-search has two mutually exclusive backend features (ripgrep default, zlob), but internal cfg gates for ripgrep-only code used cfg(not(feature = "zlob")) instead of cfg(feature = "ripgrep") — see crates/fff-core/src/ignore.rs:42, crates/fff-core/src/walk/mod.rs:16, crates/fff-core/src/constraints.rs:172 / 443 / 556 / 580. With --no-default-features neither feature is set, so the ripgrep branches were compiled but the ignore / globset deps (gated on dep:ignore / dep:globset behind the ripgrep feature) were absent, yielding 15 cascading "unresolved module or unlinked crate" errors.

Fix

• Tighten the affected gates to all(not(feature = "zlob"), feature = "ripgrep") so the ripgrep code is skipped when the ripgrep feature is off.
• Add a top-level compile_error! in crates/fff-core/src/lib.rs when neither backend feature is enabled, so the failure is a single actionable message instead of a cascade.

Steps to reproduce

Setup:

git clone https://github.com/dmtrKovalenko/fff.git
cd fff

Trigger (on pre-fix main):

cargo check -p fff-search --no-default-features

Expected: build succeeds, or cargo emits a clear diagnostic identifying the required feature.

Actual (pre-fix): 15 errors, first ones:

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `ignore`
  --> crates/fff-core/src/ignore.rs:44:9
error[E0432]: unresolved import `ignore`
 --> crates/fff-core/src/walk/ripgrep.rs:5:5
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `globset`
   --> crates/fff-core/src/constraints.rs:558:5
...
error: could not compile `fff-search` (lib) due to 15 previous errors

How verified

cargo check -p fff-search --no-default-features                    # single compile_error diagnostic
cargo check -p fff-search --no-default-features --features ripgrep # ok
cargo check -p fff-search                                          # ok (default = ripgrep)
cargo check -p fff-search --no-default-features --features zlob    # ok
cargo test  -p fff-search --lib --no-default-features --features ripgrep # 98 passed; 0 failed

Top of --no-default-features output after the fix:

error: fff-search requires either the `ripgrep` (default) or `zlob` feature. Enable one, e.g. `--features ripgrep` or `--features zlob`.

Automated triage via Gustav. Honk-Honk 🪿

[gustav-fff]

[triage-bot] CI note: Fuzz Tests (windows-latest) failed with ##[error]The operation was canceled. at 20m17s while cargo was still compiling — a job-time cancellation, not a test assertion failure (see https://github.com/dmtrKovalenko/fff/actions/runs/28899402130/job/85732131660). This job passed on the previous main run (https://github.com/dmtrKovalenko/fff/actions/runs/28888568812/job/85694982189) and the diff here (cfg gate tightening + compile_error!) does not touch the fuzz path. Rerunning the job should be sufficient.

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