layersvt: Add common layer foundation library and unit tests

[olehkuznetsov]

Implement the common layer support library (layersvt_common) as an OBJECT library to provide shared, thread-safe infrastructure across Vulkan layers:

• DeviceInstanceTracker: thread-safe VkPhysicalDevice to VkInstance mapping using std::shared_mutex with eager device enumeration (EagerMapDevices).
• DispatchTableManager: thread-safe instance and device dispatch table storage with DispatchDownstream compile-time dispatch helper and loader callback tracking.
• LayerManifest: declarative layer metadata container with downstream extension merging and Android b/143293104 compatibility workaround.
• LayerBase: standardized layer lifecycle implementation with runtime loader chain validation, teardown ordering, and extensible virtual hooks for custom command interception.
• layer_keep_alive: Android RTLD_NODELETE self-pinning with explicit EnsureLayerKeepAlive() to prevent static linker dead-stripping.
• log.h: cross-platform VT_LOGI, VT_LOGW, VT_LOGE logging macros.
• Unit test suite (test_common_layer): unit tests covering all common components, eager mapping, and lifecycle execution under a mock loader.

Bug:
Test: new tests

[emrekultursay]

I did a first round of review in good faith, but it took me much more time than I'd like to reserve for such a change.

I'd like to give a gentle warning that such changes put a high code review burden on the reviewer to verify that this is correct or makes the right set of changes. AI code review can help find interesting corner cases, but at the end of the day, I read "every single line", and ask "why was this done?" and "is this the best way?" for each of them, and there's hundreds of such questions in this PR.

For instance, when I look at this PR, the first thing that hits me is that there's lots of dead code (in fact the entire PR is dead code, it's not used by any layer). Then I look at follow-up PR which uses this in DebugMarker layer, and I notice that some methods (e.g., DispatchDownstream) I thought were dead are used there, so it's not dead, but there's still some other code (e.g., log.h) that's still dead. But will a third PR use it? That's just one small aspect.

Then there's also some questionable stuff (e.g., keep_alive change). Those deserve a conversation of their own; but get lost in such a big PR that changes too many things in one shot.

To make it more reviewer friendly (and get changes reviewed faster with less complaints), I'd recommend splitting them differently, e.g., here I'd split on a feature-by-feature basis, like this:

PR 1. Extract and empty base class. Move all layers to that base class. No-op change, takes 1 minute to review.
PR 2. Add manifest to base class constructor, move all layers to use the base class constructor.
PR 3. Add DeviceInstanceTracker to base class, move all layers to use it.
PR 4. Add EagerMapDevices, move all layers to use it. If any layer doesn't need it, add a comment instead of a call to EagerMapDevices.
PR 5. Move the keep_layer_alive functionality. Get precise code review feedaback, explore STATIC vs OBJECT.
PR 6. ...

Every single PR introduces one small functionality, comes with the unit tests for that functionality, does not introduce unused/dead code or tests.

I am guessing that's how you implemented and verified this yourself: step-by-step. Then, as the reviewer, if I see the progression in the same way, it'd be much easier for me to understand their impact, and verify.

[emrekultursay]

I did a first round of review in good faith, but it took me much more time than I'd like to reserve for such a change.

To make this PR (and the follow-up) move forward with least effort, one option is to keep this PR, but split it into various commits within the PR, where each one does that one functionality. Then I (and you and AI) can review and verify each commit separately.

[olehkuznetsov]

The PR has been reorganized into 10 cohesive, atomic commits with co-located unit tests in layersvt/test/common/:

1. DispatchTableManager (swnktrxs)
2. DeviceInstanceTracker (uozumnzo)
3. Android layer keep-alive self-pinning (txxtsnsx)
4. Downstream command dispatch templates (dispatch_downstream.h) and LayerBase routing (vyqsousy)
5. Instance and device lifecycle management in LayerBase (lkloutrx)
6. Physical device tracking and group enumeration (lzrrxwxo)
7. LayerManifest and extension enumeration (qtnzxsmp)
8. Exported C Vulkan layer entry points via layersvt_entrypoints OBJECT library (zzrypnnq)
9. Usage documentation, public utilities, and MSVC build support in layer_base.h (ozksrpkp)
10. Common layer library documentation in README.md (nquyptso)

All review comments have been addressed in their respective commits, and all presubmit checks are passing.

[olehkuznetsov]

You can start from high level review of the common layer library documentation in README.md