Fix TBC 2.5.6 debuff color compatibility

[autyan]

Problem

TBC Anniversary 2.5.6 removed the global DebuffTypeColor table. SUF indexes it in the aura, health, and highlight modules, so an aura update can abort and leave unit-frame content or debuffs stale.

A runtime probe from zhCN client 2.5.6.68775 / Interface 20506 confirms:

• DebuffTypeColor = nil
• UnitAura = function
• DEBUFF_TYPE_NONE_COLOR, MAGIC, CURSE, DISEASE, POISON, and the new BLEED color constants are tables

The UnitAura observation is an existence/type check only. It does not establish that its return tuple is semantically unchanged from 2.5.5. This PR therefore does not modify the aura query path; tuple parity should be verified separately against C_UnitAuras.GetAuraDataByIndex before introducing a wrapper.

Change

• Add ShadowUF.API.GetDebuffTypeColor as a compatibility boundary instead of recreating a Blizzard global.
• Prefer the legacy table where it still exists.
• Fall back to the individual 2.5.6 color constants, including Bleed.
• Always return the none color for unknown aura types.
• Route the three existing color consumers through the helper.
• Register the constants in .luacheckrc.
• Add Interface 20506 to both TOCs.

This keeps the patch limited to the confirmed aura-color breakage and TOC metadata.

This overlaps with #73, but additionally handles DEBUFF_TYPE_BLEED_COLOR, avoids mutating _G, makes the existing auras.lua lookup safe for an unknown non-nil aura type, and passes the repository luacheck configuration with the new globals.

Verification

• Repository luacheck configuration: 0 warnings / 0 errors across 40 files.
• luac -p passes for all changed Lua files.
• Mocked 2.5.6 path: Magic, Bleed, and unknown types return the expected colors with DebuffTypeColor = nil.
• Mocked legacy path: the existing DebuffTypeColor entries remain authoritative.

[autyan]

Additional in-client semantic check on zhCN TBC Anniversary 2.5.6.68775:

• UnitAura and C_UnitAuras.GetAuraDataByIndex were both callable.
• Sample: player/HELPFUL/1, spell ID 27044.
• The 12 legacy tuple fields consumed by SUF matched the structured AuraData fields (name through isBossAura).

This is evidence that the normal helpful-aura tuple is still compatible in 68775, but it is deliberately not presented as proof for every unit/filter combination; a target harmful sample was not captured in this run. It supports keeping this color fix independent from any broader UnitAura wrapper.