fix(component): compile kiln-component standalone + fix the isolated build (SR-54, partial #446)

[avrabe]

The bug (#446)

cargo check -p kiln-component failed to compile on clean main. default = ["kiln-runtime/std"] enabled the dependency's std but never kiln-component's own std/kiln-execution features, so cfg-gated items (link_imports, call_direct_export/call_command_entry, the runtime_engine field) vanished. Only workspace feature unification hid it — so CI stayed green while a consumer depending on kiln-component alone got a build failure.

Fix

default = ["std", "kiln-execution"]. kiln-execution is a pure cfg flag (kiln-execution = [], no deps), so no dependency graph changes — it just enables by default the execution surface a standalone consumer needs, aligning the isolated build with the workspace build.

Verified

• cargo check -p kiln-component (bare, default features) now compiles;
• cargo check --workspace still green (unification unaffected);
• consumers unchanged: kilnd 6+4+13 pass, kiln-wasi lib 68 pass.

Scope — honest boundary

This fixes the lib isolated build (the #446 headline). It does not fix the crate's test/example targets, which are separately and more deeply rotten — cargo test -p kiln-component --no-run still fails with ~47 type mismatches plus references to removed APIs (execute_start, Component.options, ExternType::Function, task_manager, async_). That's a larger fix-or-delete cleanup of stale tests; I've left #446 open with a remaining: note for it rather than pretend this closes it.

Accumulating toward v0.4.3 (rivet SR-54).

🤖 Generated with Claude Code

[codecov]

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[github-actions]

🔍 Build Diagnostics Report

Summary

Metric	Base Branch	This PR	Change
Errors	0	0	0
Warnings	5	5	0

🎯 Impact Analysis

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• 0 new warnings introduced by your changes
• 0 total errors in modified files
• 0 total warnings in modified files
• 0 files you modified

Cascading Issues (Your Changes Breaking Other Files)

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• 0 new warnings in unchanged files
• 0 unchanged files now affected

Note: "Cascading issues" are errors in files you didn't modify, caused by your changes (e.g., breaking API changes, dependency issues).

✅ No Issues Detected

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🔧 To reproduce locally:

# Install cargo-kiln
cargo install --path cargo-kiln

# Analyze your changes
cargo-kiln build --output json --filter-severity error
cargo-kiln check --output json --filter-severity warning