quantize: add option to automatically choose optimal quant types to reach a bpw target at lowest error

[EAddario]

This PR introduces a new option --target-bpw implementing an optimised quant type selection algorithm to automatically determine per-tensor quantisation types in order to achieve a target bits-per-weight (bpw), with minimal estimated quality loss.

The selection algorithm,

• builds a candidate set of quant types (K or IQ types)
• for each layer/tensor, it simulates quantise→dequantise per candidate type, and estimates error using a weighted MSE error function. If the imatrix includes activations, it adds a bias penalty term to better reflect forward‑pass impact, making the error estimation more accurate and thus the quant type selection
• it filters candidates to the pareto frontier (lowest error for a given size), then starts from the smallest bpw mix increasing to larger formats, based on the best error‑reduction per added bit, until the global bpw budget is reached
• returns a map of tensor name → ggml_type overrides, which the main quantisation pass uses. If the minimum achievable BPW already exceeds the target, it returns that minimum.

The target_bpw_type() function will look over all quantisable tensors (e.g. embedding, output, etc.) unless --output-tensor-type, --token-embedding-type, and/or --tensor-type options are also used, in which case they'll take precedence.

--prune-layers can also be used in the same run, in which case the target_bpw_type() will skip the pruned layers and only consider the remaining against the total bpw budget.

Important note:

An imatrix that includes activations is required for the algorithm to work. At the time of writing, this is only available by generating the file using #14891 with the --output-format gguf option.

Typical usage: llama-quantize --imatrix imatrix-with-activations.gguf --target-bpw 5.18 LLM-Model-F16.gguf BPW-Quantized-Q4_K_M.gguf q4_k_m

Special thanks to @ddh0 and @compilade for their contributions during the development of this PR.

PR created in draft until testing is completed