fix: migrate rquest to wreq + engineering review hardening (17 fixes)

[Zireael]

Summary

Migrates from rquest to stable wreq (v5) and applies 17 engineering-review fixes covering reliability bugs, DRY refactoring, dead-code cleanup, cache eviction, and test coverage.

Migration

• rquest → wreq: Switches HTTP client from rquest (unstable, BoringSSL conflicts) to wreq (stable v5, OpenSSL-compatible). Resolves the BoringSSL vs OpenSSL linking conflict that blocked builds on some platforms.

Bug Fixes (6)

#	Fix	Files
.4	browserless::extract_text_simple now skips <script>/<style> content	providers/browserless.rs
.5	Extract/Scrape chain uses shared deadline instead of per-call timeout, preventing budget overflow	engine.rs
.6	stealth provider maps HTTP errors as SearchError::Api instead of SearchError::Config	providers/stealth.rs
.8	finalize_response() wired into execute_search return path (result_count was 0 until run() fixed it)	engine.rs
.13	retry_request .when() now also matches SearchError::Wreq errors	providers/mod.rs
.14	Cross-platform home_dir() — resolves $HOME on Unix, %USERPROFILE% on Windows	config.rs, cache.rs, logging.rs, cli.rs
.16	Cache disk write failures now log tracing::warn! instead of silent let _ =	cache.rs

DRY Refactoring (5)

#	Extraction	From → To	Copies Removed
.9	augment_query()	brave.rs, serper.rs, you.rs → providers/mod.rs	3
.10	map_freshness()	brave.rs, you.rs → types.rs	2
.17	extract_title()	stealth.rs, browserless.rs → providers/mod.rs	2
.11	epoch_days_to_date()	logging.rs, xai.rs → utils.rs (new)	2
.2	try_provider() / try_provider_remaining()	inlined boilerplate → helper functions in engine.rs	~160 lines

Cleanup (2)

• .12: Removed unused Provider::timeout() trait method + 12 provider impl blocks + 11 unused Duration imports
• .19: Removed build_providers() call from execute_special — now only constructs needed providers per mode

Enhancements (1)

• .15: Cache file eviction on startup — scans q_*.json files, removes expired (≥5 min old) and unparseable entries

Test Coverage

59 unit tests + 36 integration tests = 95 total (was 46+36=82)

Area	Tests Added	What's Covered
classify.rs	+13	social, news, academic, scholar, patents, people, extract, similar, images, places, general, priority, SE-focused (12 sub-tests)
engine.rs	+5	normalize_url edge cases (trailing slash, query params, fragments), provider_allowed with/without filter
browserless.rs	+4	extract_text_simple script/style skipping, visible text preservation
you.rs	+5	YouResponse deserialization (hits-only, news-only, empty, optional fields)
cache.rs	+8	should_cache_query_response (success, all_providers_failed, degraded-empty, empty-no-failures), path determinism, mode sensitivity, case insensitivity, q_ prefix

Verification

$ cargo test      → 95 passed, 0 failed
$ cargo clippy --release → 0 warnings