Return the constrained VBR reservoir underflow

[thomas-vilte]

Description

When the constrained VBR reservoir becomes negative, the reference returns those bits to the current frame (celt_encoder.c):

if (st->constrained_vbr && st->vbr_reservoir < 0)
{
/* We're under the min value -- increase rate */
int adjust = (-st->vbr_reservoir)/(8<<BITRES);

nbAvailableBytes += silence?0:adjust;

st->vbr_reservoir = 0;

}

pion would simply clamp to zero and lose them. A negative reservoir means that previous frames spent less than the target rate, so this loop is precisely what keeps the average at the target; without it, the mode falls short and never recovers.

Measurement

Constrained VBR over 60 seconds of music, against opus_demo restricted-celt -cvbr:

bitrate	SNR before	SNR after	bytes pion before	after	libopus
24000	−0.208	+0.017	57.50	61.00	61.0
48000	−0.101	+0.007	115.88	121.01	121.0
96000	−0.224	+0.007	235.06	241.02	241.1

The other modes remain unchanged, verified: CBR 7.0676 and 15.0721 dB, unrestricted VBR 6.3886 and 15.0756, identical to before the change.

Mode Status

With this, all four paths are now on par with the reference on the same material:

mode	24 kb/s	48 kb/s	96 kb/s
CBR	+0.014	+0.015	−0.002
VBR	+0.012	+0.003	−0.005
CVBR	+0.017	+0.007	+0.007
mono VBR	+0.001	−0.005	−0.044

Reference issue

Part of #34.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.10%. Comparing base (9bfb54d) to head (533509a).

Files with missing lines	Patch %	Lines
internal/celt/encoder.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   93.11%   93.10%   -0.01%     
==========================================
  Files          58       58              
  Lines       10408    10411       +3     
==========================================
+ Hits         9691     9693       +2     
- Misses        505      506       +1     
  Partials      212      212              

Flag	Coverage Δ
go	93.10% <75.00%> (-0.01%)	⬇️

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