D306/D308: investigate z-fighting — depth precision == N64, real fighting confirmed; GE_ZF flicker probe in-tree

docs/porting-notes.md

@@ -1201,6 +1201,12 @@ produce the same visual symptom but need opposite fixes.
 
 **Update 2 (same investigation, immediately after): freezing a *value* and freezing a *pointer target* are different operations — treating them the same crashed twice.** Extending the same freeze technique to `render_pos` (a `Model*` field, unlike `field_488.pos` a plain value) by skipping the call that reassigns it (`subcalcmatrices`) seemed equivalent to M-167's "skip the write" but wasn't: `render_pos` is a pointer into a per-tick dynamically-reallocated arena, and skipping its reassignment only works if the pointer it's left holding is still valid — which depends on model lifecycle, not just on this tick's logic. Two fixes in a row tried to detect "is it safe to skip" by inspecting either the pointer's *value* (a sentinel check, `!= NULL && != -1`) or a *cached identity* (comparing the pointer against a remembered "already validated" pointer) — both crashed, because **the allocator reuses model-pointer addresses across genuinely different logical model instances**: a freshly-`CREATE`d model can land at the exact address a previously-removed model occupied, so both the sentinel check and the identity check wrongly conclude "this is already valid" for an instance whose `render_pos` is still an uninitialized tombstone. This is the same "don't guess validity from a value that can coincidentally look right" trap as the paragraph above, encountered a second time from a different angle (a *pointer's* target identity, not a *field's* content). **The robust fix:** never skip the call that establishes the pointer's validity — let it always run — and apply the freeze as a *contents* copy afterward, on top of a buffer that's now unconditionally guaranteed valid. If you must "hold something steady" and that something is reached through a pointer that some other code reassigns, freeze the *pointee's bytes*, not the *act of assignment* — the assignment is usually also the thing keeping the pointer sane.
 
+## D12. Depth precision: the port matches N64 exactly — z-fighting seen on PC is probably faithful (D306/D308)
+
+When a user reports z-fighting, do NOT assume the software renderer has less depth precision than the RDP. Verified for GE: the RSP's vertex transform (`rsp/graphics/gmain.s`) writes post-transform x/y/z/w back to the vertex buffer as **full 32-bit 16.16 fixed-point** (`sdv` stores), not s16 8.8 — the RDP interpolates from full-precision NDC and quantizes once, into its 24-bit z-image. The port keeps floats through `gfx_sp_vertex` and writes a `GL_DEPTH24_STENCIL8` buffer: same or better precision at every stage, same near/far. So coincident geometry authored into the level fights on both platforms; a PC-only explanation needs positive evidence (an N64 reference of the same spot), not just the existence of the symptom. A mitigation like switching to 32-bit float depth would *reduce* fighting but diverge from N64 behavior — that's a rule-2-style decision, not a silent fix.
+
+**Probe technique that worked (vs. three variants that didn't):** detecting z-fighting from emitted triangles requires filtering out (a) tessellation gradients — adjacent tris of one surface have near-equal depths by construction — and (b) camera-motion coincidences — as the camera moves, unrelated world points pass through the same screen cell and their depths happen to sit close. What survives both: a coarse screen-cell grid keeping a short per-cell depth history, flagging only **non-monotonic** (a-b-a) oscillation within a small tolerance that **recurs many times at the same cell**. Monotonic drift = gradient; one-off close pairs = motion artifact; sustained recurrence = a real fight. `GE_ZF` in `port/fast3d/gfx_pc.cpp` implements this (`GE_ZFTOL` tolerance, `GE_ZFBOX` region filter).
+
 ## E. Process / method notes
 
 - Investigation loop is: reproduce → env-gated capped probe → root-cause