docs: giant FFT plan generator analysis (notes_giant_fft.md)

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+# FUN_18002f980 (giant) — recursive FFT plan/permuation generator
+
+Signature: `ulonglong *FUN_18002f980(plan*, log2, src, flags, depth, uVar2, scratch)`
+
+Role: builds the FFT stage plan (bit-reverse ordering + sign-flip masks),
+NOT a numeric executor (no FP math inside; all integer indexing).
+
+## Factor split (top)
+```
+iVar2 = DAT_181c5e0fc[log2]      // per-log2 factor (4/8/9/13... from static table)
+iVar19 = log2 - iVar2            // reduced log2 for subproblem
+if iVar19 < 0x12:  pu = FUN_18002e360(iVar19, scr)   // small-plan path (radix2/3 generator)
+else:             pu = FUN_18002f980(plan, iVar19, scr, ..., depth+1)   // recurse
+*(plan + 0x78 + depth*8) = pu   // store sub-plan ptr per depth
+*(plan + 0x38)      = param_6   // (entry stores callback/context)
+*(plan + 0x1c)      = (1<<iVar19)*0x10 + 0x3f & ~0x3f   // size budget
+```
+
+## Stage iteration
+```
+iVar25 = 1 << iVar19
+uVar3  = DAT_181c5e15c[log2]     // 2nd factor table (stage exponent bundle)
+iVar33 = 1 << (uVar3 - iVar2)    // butterfly count/base
+loop groups:
+  for (uVar26=1; ...) {
+    iVar44 = 1 << (iVar16+2);  iVar32 = 1 << (iVar16+2 + dirflag)
+    if (iVar12 == log2) -> radix-8 permute block
+      local_900[k]=src[±idx*8]  (and ^= DAT_181c5e1e0 for negative/reflected)
+      processed 16 qwords = 8 complex pairs; groups x butterflies
+    next stage: uVar26 <<= 2; iVar16 -= 2            // radix-4 sweep
+    else branch radix-3: uVar26 <<= 3; iVar16 -= 3
+    ...
+  }
+while (iVar12 < log2)
+outer while (uVar24 < (iVar25-1+iVar33)/iVar33)
+```
+
+## Key masive consts
+- DAT_181c5e1e0  = sign-flip XOR mask (bit for im part) applied to mirrored indices.
+- DAT_181c5e0fc / 181c5e15c / 181c5e3fc = per-log2 factor tables (read as int, index log2).
+- Base small planner FUN_18002e360: iVar6=1<<log2; if <9 return; else select radix2/3
+  for log2==4 || >10 -> 2 else 3; loops similarly, returns param_4 (plan tail).
+- Return: puVar10 + (1<<log2)*2  (aligned plan tail after current stage block).
+
+## Notes for reimplementation
+- Exact ordering must be transcribed from the index algebra (the branches accumulate a
+  combination of ± vs complemented indices; XOR selects conjugate/negate for 2x recalc).
+- The 7 giants are per-(data type, direct/inverse, maybe r2c/c2r) variants (identical
+  structure; different base-call FUN_18002e360 vs FUN_180023860).
+- Stage kernels executed at runtime are the dispatched vector kernels (see roadmap),
+  driven from this plan's ordering; the transform itself therefore reproduces a
+  specific FFT: split-radix 2/4/8-style in-place butterflies with fused reorder.