fix(ir): preserve layouts and symbolic slice dimensions

[zhen8838]

Why

• Reshape dropped expressible broadcast sharding, while symbolic slice endpoints depended on authoring form and broke packed-cache call boundaries.

What

• Preserve bare fully-Broadcast ShardLayout through reshape.
• Centralize dimension normalization in isl and accept bare symbolic slice endpoints.

Contract

• hir.md: Reshape carries expressible fully-Broadcast layouts; Slice sizes use normalized symbolic dimensions.
• types.md §4: IR type/attribute dimensions share one normal form, and dim-arithmetic Calls remain composable.
• parser.md: compile-time dimensions, including an axis extent, are valid slice endpoints.

Risk

• ComposedLayout(outer=ShardLayout) remains outside the reshape rule. Symbolic divisors and min/max may stay unnormalized. Printed dimensions use canonical forms rather than authored helpers. CPU-side cost is +1–2%.