Upgrade Fix64/UFix64 to use github.com/onflow/fixed-point library

[devin-ai-integration]

Closes #4127

Description

This PR upgrades Fix64Value and UFix64Value to use the github.com/onflow/fixed-point library, following the same pattern as Fix128Value/UFix128Value. This is a draft PR for initial review.

Link to Devin run: https://app.devin.ai/sessions/3d6a099b4b7b4a5c84c4162298b7922f
Requested by: [email protected]

Key Changes

1. Type definitions (interpreter/value_fix64.go, interpreter/value_ufix64.go):

   • Changed from type Fix64Value int64 to type Fix64Value fix.Fix64
   • Changed from type UFix64Value struct { UFix64Value values.UFix64Value } to type UFix64Value fix.UFix64
   • Updated all constructors to use func() fix.Fix64 and func() fix.UFix64 callbacks
   • Arithmetic operations now call library methods: .Add(), .Sub(), .Mul(), .Div(), .Mod()
   • Comparison operations now use: .Lt(), .Lte(), .Gt(), .Gte()

2. Added utility functions (fixedpoint/convert.go, fixedpoint/check.go):

   • Fix64FromBigInt(), Fix64ToBigInt(), UFix64FromBigInt(), UFix64ToBigInt()
   • Fix64FactorAsBigInt and UFix64FactorAsBigInt constants

3. Updated call sites:

   • bbq/compiler/compiler.go: Cast values when creating constants
   • interpreter/decode.go: Updated CBOR decoding
   • interpreter/interpreter.go: Fixed string parsers and min/max constants
   • stdlib/account.go: Added fix.UFix64(balance) conversions for account balances

Critical Areas for Review

⚠️ This is a draft PR with limited testing - per the original request, extensive testing was deferred.

1. Type conversions in stdlib/account.go:484,510: Verify that fix.UFix64(balance) correctly converts raw uint64 balance values to the scaled fixed-point representation

2. Saturation arithmetic behavior: The old code used custom SaturatingPlus/SaturatingMinus methods; the new code manually handles overflow by returning fix.UFix64Max/fix.UFix64Zero. Verify semantics match.

3. Storage encoding/decoding: Changes to CBOR encoding in value_ufix64.go - verify roundtrip correctness

4. Arithmetic precision: All arithmetic operations now use fix.RoundTruncate - verify this matches expected behavior

5. Interface completeness: While the code compiles and implements atree.Storable, NumberValue, etc., verify all requirements are met

Testing Status

• ✅ Local build successful
• ⚠️ CI failures fixed (balance type conversions)
• ❌ Full test suite not run yet
• ❌ No new tests added

---

Checklist:

• Targeted PR against master branch
• Linked to Github issue Upgrade [U]Fix64 to new implementation #4127
• Code follows the standards mentioned in CONTRIBUTING.md
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels
• Run full test suite and verify all tests pass
• Verify saturation arithmetic semantics match old implementation
• Verify storage encoding/decoding roundtrips correctly

[devin-ai-integration]

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[github-actions]

Benchstat comparison

• Base branch: onflow:master
• Base commit: 3ec0c24

Results

	old.txt	new.txt
	time/op		delta
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	2.49ns ± 0%	2.50ns ± 0%	~	(p=1.000 n=1+1)
QualifiedIdentifierCreation/Three_levels-4	85.5ns ± 0%	85.4ns ± 0%	~	(p=1.000 n=1+1)
SuperTypeInference/arrays-4	234ns ± 0%	241ns ± 0%	~	(p=1.000 n=1+1)
SuperTypeInference/composites-4	90.1ns ± 0%	93.0ns ± 0%	~	(p=1.000 n=1+1)
SuperTypeInference/integers-4	307ns ± 0%	310ns ± 0%	~	(p=1.000 n=1+1)
	alloc/op		delta
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	0.00B	0.00B	~	(all equal)
QualifiedIdentifierCreation/Three_levels-4	64.0B ± 0%	64.0B ± 0%	~	(all equal)
SuperTypeInference/arrays-4	96.0B ± 0%	96.0B ± 0%	~	(all equal)
SuperTypeInference/composites-4	0.00B	0.00B	~	(all equal)
SuperTypeInference/integers-4	0.00B	0.00B	~	(all equal)
	allocs/op		delta
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	0.00	0.00	~	(all equal)
QualifiedIdentifierCreation/Three_levels-4	2.00 ± 0%	2.00 ± 0%	~	(all equal)
SuperTypeInference/arrays-4	3.00 ± 0%	3.00 ± 0%	~	(all equal)
SuperTypeInference/composites-4	0.00	0.00	~	(all equal)
SuperTypeInference/integers-4	0.00	0.00	~	(all equal)