Add UNCONST profile extensions and rework the tile info API

[kpchoi]

This implements the 16K small-tile support discussed in #228 as a set of OpenAPV profile extensions, together with the API changes needed to handle large tile counts. See docs/profile_ext.md for the extension definition.

UNCONST profiles

• <profile>-UNCONST extensions for all seven RFC 9924 profiles (e.g. 422-10-UNCONST, profile_idc 43), defined in a new document docs/profile_ext.md together with the numbering convention and the existing 16C12 profiles
• They follow all constraints of their base profile but have no limit on the number of tile columns/rows, and allow tiles down to one MB (16x16)
• Tile arrays in the encoder, decoder, and frame header are allocated through the instance allocator instead of fixed OAPV_MAX_TILES arrays, so the tile count is only bounded by the frame size

Tile info API rework (source-incompatible changes)

• oapv_frm_info_t carries the tile partitioning of a frame
• New oapvd_info_tile() fills caller-provided tile positions with a capacity check; the fixed 400-entry oapv_tile_info_t is removed
• oapvd_decode_frame() takes a tile index list for partial decoding, and --cyclic-tile-decoding takes the number of tiles to decode per frame
• Parameters of the reworked decoder APIs are validated against NULL and out-of-range values

Fixes

• 444-16C12 was only half supported: it could not be selected by the encoder and its bitstreams were rejected by the decoder
• OAPV_CFG_SET_DISABLE_COMPANDING was overwritten by the per-frame companding derivation; it is now kept separately and overrides it
• Bottom-boundary tiles reported an unclipped height in the tile info
• A bitstream buffer too small for the tile layout is rejected before encoding starts instead of failing mid-encode

Documentation

• New docs/programmers_guide.md covering encoding, both decoder API sets, tile-based partial decoding, runtime configuration, and the custom memory allocator; linked from the README

[m-w-park]

Looks good to me.

[ssantihub]

LGTM