Match AV1 symbol rate costs to the range coder minimum

HEIF_IMPLEMENTATION_PLAN.md

@@ -246,6 +246,33 @@ Further controller-state findings after `b2aee3036` on 2026-09-05:
   Updating CDFs after each selected block is not the same as refreshing every RD cost after that block.
   This missing cost-state lifecycle is an architectural deviation. Its contribution to time or quality has
   not been measured; no isolated cache or invented effort-dependent refresh policy has been introduced.
+- Reference `av1/encoder/rd.c:687-853` initializes the complete cost state and combines control and speed-feature
+  update frequencies. `av1/av1_cx_iface.c:391-394,1342-1348` defaults controls to superblock updates and disables
+  MV-cost updates for all-intra configuration. These are configured policies, not a public-effort mapping.
+  `av1/encoder/encoder_alloc.h:58-88` omits the main MV-cost allocation for all-intra; displacement costs are
+  initialized only when needed (`rd.c:843-851`). Worker costs are separately owned when their update policy
+  requires independent state (`av1/encoder/ethread.c:1610-1641`). Managed sizing and lifetime must follow those
+  usage boundaries rather than adding every table to every frame or candidate.
+
+CDF symbol-cost floor correction, verified after `37541f7f3` on 2026-09-05:
+
+- `Av1ProbabilityCost.cs:47-52` previously passed each CDF interval directly to raw probability conversion.
+  Native `av1/encoder/cost.c:30-48` first floors symbol mass at `EC_MIN_PROB` (4, `aom_dsp/entcode.h:21`).
+  Raw conversion has a different domain and still permits 1 (`av1/encoder/cost.h:32-42`). Conflating the two
+  overcharged sufficiently rare symbols by up to 1,024 rate units, or two bits.
+- CDF symbol conversion now applies the existing range-coder minimum before raw conversion. Gathered edge
+  partition costs (`Av1SymbolEncoder.cs:969-980,1003-1014`) use that same symbol boundary, matching native
+  `av1/encoder/partition_search.c:3419-3449`. Existing raw-probability behavior and tests remain unchanged.
+- The new zero-mass middle-interval regression failed before correction: expected 6,656, actual 7,680
+  (`symbol-floor-red.trx`). Cases at masses 0 through 4 and 8 exercise both ordinary CDF and gathered-symbol
+  conversion. They do not replace the missing RD-cost refresh lifecycle or coefficient optimizer.
+- Final Release .NET 11 build: zero errors and 1,009 existing warnings; Roslynk: zero compiler errors.
+  Serialized Visual Studio VSTest passes 2,283/2,283 in 27.0448 seconds (`symbol-floor-final.trx`), covering
+  entropy, intra-superblock, encoder-frame, and public HEIF encoder tests.
+- Optimized libaom decoding was repeated after the final edit for 23 palette, eight partition, and twelve color
+  sequence streams: 38,973 samples, maximum error 0, and zero samples exceeding one. Per-plane reports remain
+  in the temporary takeover directory. These are bounded same-bitstream checks, not separate-encoder parity,
+  a quality improvement claim, or performance acceptance. No benchmark was run.
 
 Palette coded-boundary correction, verified after `b2aee3036` on 2026-09-05:
 

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1ProbabilityCost.cs

@@ -48,7 +48,19 @@ internal static class Av1ProbabilityCost
     {
         int inverseLower = symbol == 0 ? Av1Distribution.ProbabilityTop : (int)distribution[symbol - 1];
         int inverseUpper = (int)distribution[symbol];
-        return GetProbabilityCost(inverseLower - inverseUpper);
+        return GetSymbolCost(inverseLower - inverseUpper);
+    }
+
+    /// <summary>
+    /// Gets the fixed-point cost of an entropy-coded symbol with a Q15 probability.
+    /// </summary>
+    /// <param name="probability">The Q15 probability numerator.</param>
+    /// <returns>The rate cost in 1/512-bit units.</returns>
+    public static int GetSymbolCost(int probability)
+    {
+        // The range coder reserves a minimum interval even when CDF adaptation collapses a symbol's mass.
+        // RD costs use that same floor; the raw probability conversion below retains its separate numerical domain.
+        return GetProbabilityCost(Math.Max(probability, Av1Distribution.ProbabilityMinimum));
     }
 
     /// <summary>

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1SymbolEncoder.cs

@@ -973,7 +973,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
             blockSize,
             context);
 
-        return Av1ProbabilityCost.GetProbabilityCost(
+        return Av1ProbabilityCost.GetSymbolCost(
             partitionType == Av1PartitionType.Split
                 ? frequency
                 : Av1Distribution.ProbabilityTop - frequency);
@@ -1007,7 +1007,7 @@ internal sealed class Av1SymbolEncoder : IDisposable
             blockSize,
             context);
 
-        return Av1ProbabilityCost.GetProbabilityCost(
+        return Av1ProbabilityCost.GetSymbolCost(
             partitionType == Av1PartitionType.Split
                 ? frequency
                 : Av1Distribution.ProbabilityTop - frequency);

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1EntropyTests.cs

@@ -156,6 +156,22 @@ public class Av1EntropyTests
         Assert.Equal(1024, Av1ProbabilityCost.GetSymbolCost(distribution, 2));
     }
 
+    [Theory]
+    [InlineData(0, 6656)]
+    [InlineData(1, 6656)]
+    [InlineData(2, 6656)]
+    [InlineData(3, 6656)]
+    [InlineData(4, 6656)]
+    [InlineData(8, 6144)]
+    public void SymbolCostUsesRangeCoderMinimumProbability(uint probability, int expected)
+    {
+        // A middle interval can collapse during adaptation. Libaom cost.c floors its mass at EC_MIN_PROB=4,
+        // giving 13 * 512 rate units at and below that floor, while mass 8 costs 12 * 512 units.
+        Av1Distribution distribution = new(16384, 16384 + probability);
+        Assert.Equal(expected, Av1ProbabilityCost.GetSymbolCost(distribution, 1));
+        Assert.Equal(expected, Av1ProbabilityCost.GetSymbolCost((int)probability));
+    }
+
     [Theory]
     [InlineData(0, 0)]
     [InlineData(1, 512)]