feat(lsp): default rust-analyzer to the full enrichment sweep

[zzet]

What

Adds a per-server DefaultSweepMode to ServerSpec and sets it to full for rust-analyzer, so Rust gets the full enrichment sweep out of the box while every other language keeps the demand-gated default.

New sweep-mode precedence (highest first): GORTEX_LSP_SWEEP env → semantic.lsp_sweep config → per-server spec default → global demand. Both operator sources still win, so semantic.lsp_sweep: demand (or the env) puts rust-analyzer back on the demand gate.

Why

The demand gate skips a file whose declarations are already statically resolved, on the premise that its call edges are recoverable without the language server. That premise holds for Go / TS / Python but not Rust: measured on ripgrep, Rust's method calls bind overwhelmingly to standard-library receiver types (Vec/String/Option/&[u8]) that the graph never indexes, so static resolution can't recover them and rust-analyzer's net-new call-hierarchy edges come precisely from the sweep the demand gate skips.

Concretely, across a language matrix (baseline vs demand vs full):

• Go / TS / Python: demand mode trims a negligible-to-small share of net-new edges (−3% / −2% / −10%) for a 1.5–2.4× enrichment speedup — demand is the right default.
• Rust: demand mode trims ~23% of rust-analyzer's net-new call edges while saving only ~5% wall time (rust-analyzer's cost is dominated by reference/hover volume the gate doesn't reduce, not the incoming calls it gates). So Rust pays the most recall for the least speed under demand — full is the better default.

A separate investigation confirmed this gap is not closable by improving Gortex's native Rust resolution: the residual ambiguity is standard-library-shaped, which is inherently language-server territory. So defaulting rust-analyzer to full is the right lever.

Tests

• resolveSweepMode precedence including the new spec-default level (env > config > spec default > demand); unrecognised spec default falls through.
• effectiveSweepMode: a spec DefaultSweepMode is honoured, the router field and env both override it, and the real rust-analyzer registry spec resolves to full.
• go build, go test -race ./internal/semantic/lsp/, golangci-lint all green. docs/lsp.md updated with the new precedence and rust-analyzer note.