Add relative content resolution

[samuel-williams-shopify]

Summary

• add a <relative:...> namespace for resolving content from the logical node request path
• preserve existing <content:...> resolution relative to the physical template location
• fall back through parent directories when no nested relative override exists
• document the new namespace and add nested override, root fallback, and physical-anchor coverage

Verification

• focused relative content tests: 2 passed
• RuboCop: 141 files inspected, no offenses
• full test suite: 358 passed, 4 errored out of 362; the four command-generation tests failed because their subprocesses could not invoke a compatible bundle executable