Cleanup PD0 paths

Description

The primary objective of this merge request is to decommission the legacy "regular" PD0 partitioning pass and route all PD0 processing through the light path with appropriate changes, across every mode, content bit depth, and super-block size.

The result replaces the previous two-path PD0 with a single, simpler path.

CLN_PD0 Remove the regular and high-bit-depth PD0 paths so that all pictures run the light PD0 path. The unification covers:

• All modes (RA, RTC, Allintra).
• 8-bit and 10-bit content: For 10-bit, PD0 now runs through the 8-bit light path while the final pass remains full bit depth.
• 64×64 and 128×128 super-blocks: 128×128 uses the same light path as 64×64, with the necessary size adjustments.

OPT_PD0_RTC_M7 Adopt a new pic_pd0_lvl for all M7 frames for 360p resolutions and lower.

CLN_RENAME_PD0 Rename the PD0 light identifiers and kernels for clarity ("light pd0 → pd0", "very light pd0 → light pd0"). (lossless)

Author(s)

@tahabenromdhane24

Performance impact

• quality
• memory
• speed
• 8 bit
• 10 bit
• N/A

Test set

• obj-1-fast can be found here
• other
• N/A

Merge method

• Allow the maintainer to squash and merge when PR is ready to create a 1-commit to the master branch. The maintainer will be able to fix typos / combine commit messages to create a more readable 1-commit message or use whatever is stated in the 'Description' section
• I will clean up my commits and the maintainer shall use 'rebase and merge' to the master branch