Add NCLX item property reader

[gregbenz]

Problem

The generic raw-property getter cannot read a recognized colr property after libheif parses it
into a typed box. The existing image-profile getters also require an image handle, so callers
cannot retrieve NCLX signaling attached to a non-image derived item.

Fix

Add heif_item_get_property_nclx_color_profile() to read an NCLX profile from a specific item
property. The returned profile uses the existing allocated-profile ownership convention and must
be released with heif_nclx_color_profile_free(). Invalid properties return an error and clear the
output pointer. As with the existing typed-property getters, property ID 0 selects the first
matching colr property.

Tests

Locate the parsed colr property on the existing AVIF fixture without assuming a property index,
verify that the raw-property getter rejects it, and check the exact NCLX values including full-range
signaling. Also discover an ispe property and verify that the new accessor rejects it while
resetting the output pointer. Finally, verify that an out-of-range property ID returns
heif_error_Usage_error / heif_suberror_Invalid_property and clears the output pointer.

[fallenmi]

Reviewed exact head 149d9249b39616193bff9820eff60566143cfaf0 against exact base/current master 2bc82b493dd8896fab3226f01977c7ac9d2ea3b8; I found no blocking issue.

The new accessor follows the existing allocated-NCLX ownership path, clears the caller's pointer on every tested error, and the added return fixes the shared typed-property bounds check before indexing. An independent public-ABI oracle on examples/example.avif read the expected NCLX values (cp=2, tc=2, mc=6, full range) by both the explicit property ID and property ID 0. It also verified exact error behavior for a non-colr property, UINT32_MAX, an invalid item, and null arguments. On the existing ICC-only tests/data/rainbow-451x461.heic, both access routes return Color_profile_does_not_exist and leave the output null.

The causal base comparison is clean: the new exported symbol is absent there, and the same out-of-range call through the pre-existing raw-property API reaches the missing-return bug and terminates with SIGBUS; exact head and GitHub's synthetic merge 345f3b433fdcc323d3b53264fc98b79678f30906 instead return Usage_error / Invalid_property. The submitted item_properties test passed ten consecutive runs on both head and merge. The related image_description_metadata and entity_groups tests passed three runs on base, head, and merge; extended_type was skipped under the deliberately codec-minimal local build. git diff --check is clean, and all 41 published checks are green.

This review was prepared with OpenAI Codex assistance under the account owner's authorization; I built and exercised the exact revisions and verified the code paths, outputs, policy, live head/base/merge state, CI, and interaction state.