Extend the UMS manifest

Define the project problem before adding syntax. A target field, package field, dependency rule, or toolchain component should have one semantic owner in the UMS model.

The path from text to command is explicit:

umsDiscoverManifest() → umsLex() → umsParse() → umsLoadText() → model validation → umsBuildPlanCreate() → project command execution.

Place the change at the earliest stage that needs new information. A new spelling touches the parser; a new constraint over existing fields may require only model validation; a different ordering rule belongs only in the plan.

Implementation path

1. Extend ums/parser/token.psm, lexer, AST, and parser.
2. Lower syntax into a typed project, workspace, dependency, or target model.
3. Validate incompatible combinations before producing commands.
4. Update discovery and resolution when the feature affects paths or dependencies.
5. Teach build_plan.psm the ordering and native command consequences.
6. Add a canonical writer representation if generated manifests need the field.
7. Emit source-located diagnostics for invalid input.

ums/parser/lexer.psm produces UmsToken values with source positions. The parser builds syntax nodes; umsLoadText() lowers them into UmsProjectModel, targets, dependencies, commands, and the workspace. Constructors such as umsTarget(), umsLinkInput(), umsCommand(), umsCommandStep(), and umsCommandArgument() keep model creation consistent. Lookups use umsTargetFind(), umsDependencyFind(), and umsCommandFind() rather than duplicating name rules.

Validation reports through umsDiagnosticAdd() and prints with umsDiagnosticsPrint(). Give each failure a code, source path and span, and explain the incompatible fields. Do not postpone an invalid manifest until Clang or the linker emits a less local error.

umsBuildPlanCreate() translates the validated model into ordered work. If a field affects native linking, preserve its semantic kind through UmsLinkInput instead of flattening it into a shell string. Command steps distinguish executable, arguments, forwarding, and environment so platform quoting remains an execution concern.

Preserve the stable bootstrap host block understood by an installed parent compiler. If older parents cannot safely ignore the new syntax, make the incompatibility explicit and provide a migration path.

umsProjectHost() intentionally reads only the stable toolchain portion required to find .prismio/build/debug/prismio. dispatchToUmsHost() may execute before the current compiler can parse the rest of a newly extended manifest. Keep this route backward-readable or bump the minimum parent requirement and make the failure actionable.

Test successful parsing, round-trip writing where relevant, validation failures, path normalization, graph ordering, generated commands, host forwarding, and platform-specific quoting. Document whether the capability affects build planning only or also changes compiler source resolution.

Add fixtures under ums/tests/fixtures and assertions in ums/test_ums.psm. Exercise umsManifestAddDependency() when the canonical writer owns the new field. Then run project-level tests through prismio build, run, test, and any custom command so the compiler-facing src/project/ums_cli.psm path is covered as well as the model in isolation.