feat: template fallback operator ${expr || 'default'}

[hardyjosh]

Caution

Stacks on #2549. Merge that first.

Motivation

Strategy field names and descriptions in .rain YAML reference token properties via templates like \${order.inputs.0.token.symbol} per \${order.outputs.0.token.symbol}. The webapp resolves these at runtime once tokens have been selected. Any other consumer (notably --describe) that renders these strings before a token is picked sees the raw placeholder, which is confusing.

Dropping the placeholder in un-selected contexts would lose the structural information. Hard-coding a substitution in every consumer is brittle and doesn't match webapp semantics.

Solution

Extend the template syntax with an optional fallback literal:

${path}                    — current behaviour
${path || 'fallback'}      — substitute literal when the path resolves to a missing token
${path || "fallback"}      — double quotes also accepted

The fallback only kicks in on PropertyNotFound("token") (i.e. select-token not yet selected). Other resolution errors still propagate. Templates without || behave identically to today — backwards compatible.

The webapp gets no regression: when tokens are selected the left-hand side always resolves and the fallback never fires. --describe (and any other "no context" consumer) gets readable output for free once registry strategies adopt fallbacks.

Registry adoption is tracked in ST0x-Technology/st0x.registry#9.

Checks

• made this PR as small as possible
• unit-tested any new functionality
• linked any relevant issues or PRs
• included screenshots (if this involves a front-end change)

[hardyjosh]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: add --describe flag to strategy-builder CLI #2548
• feat: template fallback operator ${expr || 'default'} #2551 👈 (View in Graphite)
• feat: add --tokens flag to list deployment-specific tokens #2549
• feat: add interactive mode to strategy-builder CLI #2546
• feat: add strategy-builder CLI subcommand #2544
• Consolidate JS API gui module into common crate and rename GUI to OrderBuilder #2479
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

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