console: per-value platform gating for enum cvars + cross-cvar dependency warnings

[havokentity]

Context

Follow-up to PR #159 (cvar UX: CVAR_PLATFORM_MAC / CVAR_PLATFORM_WIN flag bits + 0-as-off enum alias). That PR added platform gating at the CVAR level. This issue adds two missing dimensions of UX hygiene to the same code area:

1. Per-value platform gating inside enum cvars — the cvar itself is cross-platform but specific allowed-values aren't.
2. Cross-cvar dependency warnings — when the user sets a cvar whose prerequisite condition isn't met, print a clear hint instead of silently no-op'ing.

Part 1: per-value platform gating

The cvar is cross-platform but its enum has platform-only values:

• r_denoiser:
  • Mac: none, svgf_basic, svgf_atrous, metalfx, svgf_basic_metalfx, svgf_atrous_metalfx
  • Win/Linux: none, svgf_basic, svgf_atrous, nrd_relax, nrd_sigma, optix_hdr, optix_temporal_hdr, optix_hdr_aov, optix_temporal_hdr_aov
• r_backend:
  • Mac: metal, vulkan, software
  • Win/Linux: vulkan, software (no metal)
• r_software_blit:
  • Win: vulkan, gdi
  • Mac/Linux: vulkan (gdi is Win-only)

Implementation sketch

Per-value platform tag in the allowed-values metadata. Concretely: where r_denoiser declares its enum in src/engine/Engine.cpp, each value gets an optional CVAR_VALUE_MAC / CVAR_VALUE_WIN / CVAR_VALUE_LINUX flag. At cvar_list / cvar autocomplete time, the console filters out values whose tag doesn't match the current platform. At cvar set time, picking a disallowed value errors with:

[error] r_denoiser=nrd_relax is Windows/Linux-only; not available on macOS.
        Available on this platform: none, svgf_basic, svgf_atrous, metalfx, svgf_basic_metalfx, svgf_atrous_metalfx

Part 2: cross-cvar dependency warnings

When the user sets a cvar whose effect requires another cvar to be in a specific state, print a clear warning + actionable hint. Currently those settings just silently no-op or produce surprising behavior.

Examples from the codebase

Cvar	Prerequisite	Hint when unmet
r_sun_elevation, r_sun_azimuth, r_sky_day	r_sky_mode procedural	r_sky_mode=astronomical: solar position derives from datetime/lat/lon, not these cvars
r_clouds_coverage, r_clouds_curl_amount, etc.	r_clouds 1	r_clouds=0: cloud march disabled
r_svgf_atrous_passes, r_svgf_albedo_demod	r_denoiser starts with svgf_	r_denoiser=none: SVGF inactive
r_smoke_max_emitters	r_smoke_enabled 1	r_smoke_enabled=0: smoke loop disabled
r_water_ior, r_water_absorption_*	scene has a MAT_WATER primitive	(warn at primitive-list time, harder to detect at cvar-set time)
r_restir_k_candidates, r_restir_temporal	r_restir 1 AND denoiser active	r_restir=0 or no denoiser: ReSTIR dispatch skipped
r_light_tree_* (future tuning cvars)	r_light_tree 1	r_light_tree=0: naive uniform NEE pick
r_voxel_size	r_voxelize_demo 1 (to actually see the effect)	r_voxelize_demo=0: voxel grids hidden
r_metalfx_* (any future MetalFX cvars)	r_denoiser is MetalFX-family	r_denoiser=svgf_atrous: MetalFX specular guidance inactive
r_optix_* (any future OptiX cvars)	r_denoiser is OptiX-family	r_denoiser=none: OptiX denoiser inactive

Implementation sketch

Extend PT_CVAR macro with an optional dependency predicate, e.g.:

PT_CVAR(r_sun_elevation, "45", "...", CVAR_ARCHIVE,
        CVAR_REQUIRES("r_sky_mode", "procedural"));

Or more generally a lambda for compound conditions:

CVAR_REQUIRES_PREDICATE([](){
    auto* m = C.FindCVar("r_sky_mode");
    return m && m->value == "procedural";
}, "r_sky_mode=procedural")

At cvar set time, after the value is written, evaluate the predicate. If false, print warning. Don't BLOCK the set — the cvar still takes the new value (so a single cfg-load script can set the dependency cvar second and have the first one apply correctly). The warning is informational.

At cvar_list time, append [inactive: requires <dep>] to the description if predicate fails.

Acceptance criteria

• r_denoiser nrd_relax on Mac errors with the platform-mismatch message + lists available values
• cvar_list r_denoiser on Mac shows only Mac values; on Win shows only Win values
• cvar set r_sun_elevation 30 when r_sky_mode=astronomical prints the warning hint
• All existing cfg files (cornell_csg.cfg, etc.) still load cleanly without spurious warnings; predicates are only evaluated after explicit user cvar set, not during cfg replay (configurable via a flag — the cfg might set the dep cvar after the dependent one)
• r_denoiser=metalfx is still PARSED on Win for portability of demont.cfg (just warns + no-ops with the "this platform doesn't support that denoiser" reason); same pattern as PR feat(console): platform-filter cvar listing + accept "0" as off alias #159's flag-bit cvars

Scope estimate

~300-500 LoC across Console.cpp / Console.h + the PT_CVAR macro + the per-cvar dependency declarations in Engine.cpp. Doctest fixture in tests/cvar_ux_test.cpp extending PR #159's coverage with platform-value-gating cases + dep-warning cases.

Out of scope

• Runtime predicate evaluation on EVERY cvar access — only at set time + list time.
• Auto-correcting / cascading sets (e.g. "you set r_clouds_curl_amount, would you like to also enable r_clouds? [y/n]") — too prompt-heavy for a console UX. The warning + hint is enough.

Dependencies

Lands on top of PR #159 (already in integration). No conflicts expected — separate code areas inside the same files.