Define configured theme palette overrides

[konojunya]

Background

Hosts need reusable organization-specific palettes without putting visual values in Stack source or introducing a separate theme-selection flag. A user-defined default must be able to replace the installed default intentionally while remaining non-recursive.

Implementation Approach

• Define validated host-configured palette overrides outside Stack source.
• Resolve extends from the installed builtin catalog before applying overrides.
• Give configured names precedence over installed catalog names, including default, light, and dark.
• Preserve STK6001 fallback behavior through the effective configured default.
• Clarify the lexical restriction on consecutive hyphens and align the normalized IR schema.
• Record the decision in ADR 0007 and add a compiler conformance fixture for an externally configured theme reference.

Testing Approach

• Validate all JSON schemas and conformance documents with check-jsonschema 0.38.0.
• Run the specification Python validators.
• Run the language package tests and package check.
• Run stack-sh/compiler conformance tests against this specification checkout.

Review Perspectives / Points of Concern

• Confirm that public catalog registration remains first-merged while host configuration may intentionally shadow an installed name.
• Confirm that default: { extends: default } unambiguously means builtin-base resolution followed by configured override registration.
• Confirm that contrast concerns remain visible without silently rewriting user-supplied colors.