cargo: bring in helix-term as a real dependency
fresnel needs helix-term's MappableCommand roster and Keymaps dispatcher.
Its build.rs otherwise fetches and compiles ~300 bundled tree-sitter
grammars, one of which reproducibly fails to link on nix's toolchain under
the concurrent build. Set HELIX_DISABLE_AUTO_GRAMMAR_BUILD=1 at the
workspace level to skip that step — fresnel drives syntax highlighting
through Pierre's Shiki path in Phase 2 and its own tree-sitter for
syntax-aware collapse, so the bundled grammars aren't needed.
Also set DEVELOPER_DIR so nix's gcc can find dsymutil for debug-info
bundling on macOS.