feat: add built-in metrics and Prometheus exporter

[noneandundefined]

Summary

• implement issue [Feature]: Built-in metrics and Prometheus exporter #44 with optional per-server metrics
• add thread-safe cHTTPX_ServerMetrics() snapshots
• add one-call Prometheus exporter registration with cHTTPX_MetricsRoute()
• collect request/in-flight, connection, status-class, byte, latency, parser, timeout and rate-limit metrics
• aggregate route metrics by registered route template rather than raw request paths
• add concurrent integration coverage and a snapshot overhead micro-benchmark
• add monitoring example and EN/RU metrics documentation
• remove the standalone benchmarks/ directory and compression benchmark target
• make GitHub Pages generate module content from canonical docs/<module>/README.md files instead of keeping duplicate markdown under docs/site/content
• expose TLS, compression and metrics documentation on the website
• fix automatic SemVer calculation so feat: bumps minor, fixes/maintenance bump patch, and breaking changes bump major
• fix release build dependencies for the standard zlib-enabled build

SemVer behavior

Examples:

• v1.4.6 + feat: ... -> v1.5.0
• v1.5.0 + fix: ... -> v1.5.1
• v1.5.1 + feat!: ... or BREAKING CHANGE: -> v2.0.0

Metrics

chttpx_config_t config = cHTTPX_DefaultConfig();
config.metrics_enabled = true;

chttpx_serv_t *server = cHTTPX_AppServer(&app, "api", &config);
chttpx_router_t router = cHTTPX_RoutePathPrefix(server, "");

cHTTPX_MetricsRoute(&router, "/metrics");

Metrics stay disabled by default.

Closes #44