Add link modes: consume a dependency as a Zig import, not just a C-ABI link

[godofecht]

What

Adds a link field to #Module, one of "abi" (default) or "import".

• "abi" is the current behaviour: the module is a separately compiled artifact linked over the C ABI (pub export fn / extern fn). Required for shared libraries and C/C++ interop.
• "import" merges the module into each dependent as a plain Zig module reached with @import("name"). One compilation, no separate artifact, no link step.

The default stays "abi", so every existing project builds unchanged. A shared module is forced to "abi" by the schema.

Why

Every deps edge was a C-ABI link, so a pure Zig-to-Zig graph paid for one artifact and one link step per module, plus a build graph Zig re-validates step by step each build. That overhead scales linearly with the module count.

Measured through the real CUE -> build_spec.zig -> zig build pipeline on a 150-module graph:

operation	abi	import
clean build	12.7s	5.2s
no-op build	4.5s	0.3s
one-module change	4.9s	0.5s

import mode is ~10x faster on incremental builds and stays flat as the module count grows, because the subgraph is a single compilation that Zig caches and links once.

Changes

• schema.cue: #Link and the link field, shared pinned to "abi".
• export.cue, gen_build_spec.sh: thread link through to the generated spec.
• build.zig: module per declared module, compile step only for real artifacts, wire import deps with addImport and abi deps with linkLibrary, install only real artifacts.
• build_spec_test.zig: two invariants (shared is abi; an import dep is static).
• examples/05-import-mode/: a runnable import-edge project.
• docs/WIKI.md, README.md: document link and the source contract.
• Sync the shared schema.cue / gen_build_spec.sh into examples 01-04.

Verification

• Root zig build unchanged; zig build test 58/58 pass (2 new).
• All five examples build (Zig 0.14.0 locally; CI covers 0.14.1 / 0.15.2 / 0.16.0).

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