[trsonic]

Fixes #53: in a binaural processing group with both head-locked and world-locked loudspeaker or object elements, channels were encoded with other channels' positions.

• One encoder per pass (renderer/processing_group.{h,cc}). ProcessingGroup keeps an EncoderPass (encoder and input buffer) for the world-locked and for the head-locked channels. Each holds only its pass's channels as sources, in element order, so input i of a pass is encoded with the position of the pass's i-th channel, and the passes no longer share inputs or interpolation state. UpdateAmbisonicEncoder() sizes and sets both.
• SetElementHeadLocked() (renderer/obr_impl.{h,cc}) calls UpdateAllAmbisonicEncoders(), because a change moves the element's channels to the other pass. Its documentation says that it can allocate when an encoder's source count changes.

A group whose elements are all world-locked or all head-locked uses one encoder with the same sources as before, so its output does not change. With the reproduction from #53, both runs now render element 2 on the right: L 6.577, R 227.456.

Tests:

• tests/examples/test_binaural_mixed_lock.c renders, through oar.h, a world-locked object beside a silent head-locked one and requires the output of the world-locked object alone, and the reverse with the head-locked object alone.
• ObrImplTest.TestSetElementHeadLockedUpdatesEncoders sets a configured group's element head-locked, and back to world-locked so that the head-locked pass is empty, and requires the output of the world-locked object alone.

Both fail on main.

Verified locally on macOS arm64:

• CMake with the workflow's commands, ctest --test-dir build -L 'oar|obr' --output-on-failure: 40 of 40 pass, in Release and with -fsanitize=address.
• Bazel 8.5.1: //tests/examples:all and @obr//obr/renderer/tests:all pass (21 pass, 1 skipped). The standalone Bazel build fails on this machine on main too, with the Xcode SDKSettings.json "absolute path inclusion" error in untouched files, so these targets were built as modules from a workspace that registers apple_support's toolchain.

Closes #53

[trsonic]

Rebased onto main after #55 merged (551f974). #55 and this PR both change renderer/processing_group.{h,cc}. In the resolution, UpdateEncoderPass() reserves the index list of its own pass (the world-locked or the head-locked one) for that pass's source count, where #55's UpdateAmbisonicEncoder() reserved both lists for the group's total, and Process() fills each list within its pass's capacity as in #55. #55 already removed the unfiltered GetAmbisonicEncoderSourceChannelIndices() overload, so I dropped that sentence from the commit message and the description. The test registrations in tests/examples are appended after #52's and #58's. Locally 40 of 40 tests pass in Release and with AddressSanitizer, and the Bazel tests //tests/examples:all and @obr//obr/renderer/tests:all pass (21 pass, 1 skipped).