ADR 0014: Track tile definedness per element

Context

Allocation deliberately leaves tile payload undefined. A single contents_defined flag previously changed to true after any element write, which made every other element in the valid region readable. Whole-tile arithmetic, reductions, and payload-dependent legality checks could therefore observe carrier values that no architectural operation had produced.

The architecture needs both element-precise reads and an efficient legality predicate for operations that consume an entire valid region.

Decision

• Every TileInfo has one definedness bit for each executable-model payload element and a count of defined elements in its current valid region.
• contents_defined is the maintained summary that every element of the valid region is defined. It is not an independent source of truth.
• An element write defines only its addressed element. Rewriting an already defined element does not increment the valid-region count.
• A generic element read requires the selected element's definedness bit. It does not gain access to sibling elements merely because one write occurred.
• Operations that overwrite their complete destination valid region mark that region defined after all payload effects. Partial update operations require the untouched destination region to be defined before execution.
• Decoded operations that consume a whole source valid region reject an incomplete source before the first destination effect. Payload-dependent legality checks likewise fail closed on incomplete inputs.
• Allocation, reconfiguration, release, and reset clear all element definedness and the valid-region count. Descriptor-and-payload copies carry the corresponding definedness state.

Elements outside the valid region remain unobservable unless an operation explicitly defines and addresses them. Such writes set their element bit but do not contribute to the valid-region summary.

Consequences

One-element initialization can be tested without accidentally defining a whole tile. Reductions and other whole-region consumers cannot expose unwritten payload. The ASL model retains contents_defined for concise dispatch legality, but its value is now derived and covered by per-element regression tests.